Added `debug` option for migration notices

The `debug` option was chosen because the option also exists in
Select2 4.0.0 and provides similar warnings when configurations
are detected that are known to cause issues in Select2.

Warnings will be displayed in the console if the debug option is
enabled. As there are a lot of options that have been renamed in
Select2 4.0.0, using the older version of the options will trigger
a warning in Select2 3.5.3. Options which were also removed and
have clear migration paths will also trigger a warning with the
link to the relevant documentation.

Options which have been renamed in Select2 4.0.0 are now supported
in Select2 3.5.3, allowing people to migrate their installations
over to the new options before making the final switch.

This closes https://github.com/select2/select2/issues/3483.

select2.js

@@ -936,6 +936,155 @@ the specific language governing permissions and limitations under the Apache Lic
                 });
             }
 
+            opts.debug = opts.debug || $.fn.select2.defaults.debug;
+
+            // Warnings for options renamed/removed in Select2 4.0.0
+            // Only when it's enabled through debug mode
+            if (opts.debug && console && console.warn) {
+                // id was removed
+                if (opts.id != null) {
+                    console.warn(
+                        'Select2: The `id` option has been removed in Select2 4.0.0, ' +
+                        'consider renaming your `id` property or mapping the property before your data makes it to Select2. ' +
+                        'You can read more at https://select2.github.io/announcements-4.0.html#changed-id'
+                    );
+                }
+
+                // text was removed
+                if (opts.text != null) {
+                    console.warn(
+                        'Select2: The `text` option has been removed in Select2 4.0.0, ' +
+                        'consider renaming your `text` property or mapping the property before your data makes it to Select2. ' +
+                        'You can read more at https://select2.github.io/announcements-4.0.html#changed-id'
+                    );
+                }
+
+                // sortResults was renamed to results
+                if (opts.sortResults !- null) {
+                    console.warn(
+                        'Select2: the `sortResults` option has been renamed to `sorter` in Select2 4.0.0. '
+                    );
+                }
+
+                // selectOnBlur was renamed to selectOnClose
+                if (opts.selectOnBlur != null) {
+                    console.warn(
+                        'Select2: The `selectOnBlur` option has been renamed to `selectOnClose` in Select2 4.0.0.'
+                    );
+                }
+
+                // ajax.results was renamed to ajax.processResults
+                if (opts.ajax != null && opts.ajax.results != null) {
+                    console.warn(
+                        'Select2: The `ajax.results` option has been renamed to `ajax.processResults` in Select2 4.0.0.'
+                    );
+                }
+
+                // format* options were renamed to language.*
+                if (opts.formatNoResults != null) {
+                    console.warn(
+                        'Select2: The `formatNoResults` option has been renamed to `language.noResults` in Select2 4.0.0.'
+                    );
+                }
+                if (opts.formatSearching != null) {
+                    console.warn(
+                        'Select2: The `formatSearching` option has been renamed to `language.searching` in Select2 4.0.0.'
+                    );
+                }
+                if (opts.formatInputTooShort != null) {
+                    console.warn(
+                        'Select2: The `formatInputTooShort` option has been renamed to `language.inputTooShort` in Select2 4.0.0.'
+                    );
+                }
+                if (opts.formatInputTooLong != null) {
+                    console.warn(
+                        'Select2: The `formatInputTooLong` option has been renamed to `language.inputTooLong` in Select2 4.0.0.'
+                    );
+                }
+                if (opts.formatLoading != null) {
+                    console.warn(
+                        'Select2: The `formatLoading` option has been renamed to `language.loadingMore` in Select2 4.0.0.'
+                    );
+                }
+                if (opts.formatSelectionTooBig != null) {
+                    console.warn(
+                        'Select2: The `formatSelectionTooBig` option has been renamed to `language.maximumSelected` in Select2 4.0.0.'
+                    );
+                }
+
+                if (opts.element.data('select2Tags')) {
+                    console.warn(
+                        'Select2: The `data-select2-tags` attribute has been renamed to `data-tags` in Select2 4.0.0.'
+                    );
+                }
+            }
+
+            // Aliasing options renamed in Select2 4.0.0
+
+            // data-select2-tags -> data-tags
+            if (opts.element.data('tags') != null) {
+                var tags = opts.element.data('tags');
+
+                // data-tags should actually be a boolean
+                if (!$.isArray(tags)) {
+                    tags = [];
+                }
+
+                opts.element.data('select2Tags', tags);
+            }
+
+            // sortResults -> sorter
+            if (opts.sorter != null) {
+                opts.sortResults = opts.sorter;
+            }
+
+            // selectOnBlur -> selectOnClose
+            if (opts.selectOnClose != null) {
+                opts.selectOnBlur = opts.selectOnClose;
+            }
+
+            // ajax.results -> ajax.processResults
+            if (opts.ajax != null) {
+                if ($.isFunction(opts.ajax.processResults)) {
+                    opts.ajax.results = opts.ajax.processResults;
+                }
+            }
+
+            // Formatters/language options
+            if (opts.language != null) {
+                var lang = opts.language;
+
+                // formatNoMatches -> language.noMatches
+                if ($.isFunction(lang.noMatches)) {
+                    opts.formatNoMatches = lang.noMatches;
+                }
+
+                // formatSearching -> language.searching
+                if ($.isFunction(lang.searching)) {
+                    opts.formatSearching = lang.searching;
+                }
+
+                // formatInputTooShort -> language.inputTooShort
+                if ($.isFunction(lang.inputTooShort)) {
+                    opts.formatInputTooShort = lang.inputTooShort;
+                }
+
+                // formatInputTooLong -> language.inputTooLong
+                if ($.isFunction(lang.inputTooLong)) {
+                    opts.formatInputTooLong = lang.inputTooLong;
+                }
+
+                // formatLoading -> language.loadingMore
+                if ($.isFunction(lang.loadingMore)) {
+                    opts.formatLoading = lang.loadingMore;
+                }
+
+                // formatSelectionTooBig -> language.maximumSelected
+                if ($.isFunction(lang.maximumSelected)) {
+                    opts.formatSelectionTooBig = lang.maximumSelected;
+                }
+            }
+
             opts = $.extend({}, {
                 populateResults: function(container, results, query) {
                     var populate, id=this.opts.id, liveRegion=this.liveRegion;
@@ -979,7 +1128,6 @@ the specific language governing permissions and limitations under the Apache Lic
 
 
                             if (compound) {
-
                                 innerContainer=$("<ul></ul>");
                                 innerContainer.addClass("select2-result-sub");
                                 populate(result.children, innerContainer, depth+1);
@@ -1050,7 +1198,6 @@ the specific language governing permissions and limitations under the Apache Lic
                 opts.id=function(e) { return e.id; };
             } else {
                 if (!("query" in opts)) {
-
                     if ("ajax" in opts) {
                         ajaxUrl = opts.element.data("ajax-url");
                         if (ajaxUrl && ajaxUrl.length > 0) {
@@ -2533,9 +2680,23 @@ the specific language governing permissions and limitations under the Apache Lic
 
             if (arguments.length > 1) {
                 triggerChange = arguments[1];
+
+                if (this.opts.debug && console && console.warn) {
+                    console.warn(
+                        'Select2: The second option to `select2("val")` is not supported in Select2 4.0.0. ' +
+                        'The `change` event will always be triggered in 4.0.0.'
+                    );
+                }
             }
 
             if (this.select) {
+                if (this.opts.debug && console && console.warn) {
+                    console.warn(
+                        'Select2: Setting the value on a <select> using `select2("val")` is no longer supported in 4.0.0. ' +
+                        'You can use the `.val(newValue).trigger("change")` method provided by jQuery instead.'
+                    );
+                }
+
                 this.select
                     .val(val)
                     .find("option").filter(function() { return this.selected }).each2(function (i, elm) {
@@ -2584,6 +2745,13 @@ the specific language governing permissions and limitations under the Apache Lic
                 if (data == undefined) data = null;
                 return data;
             } else {
+                if (opts.debug && console && console.warn) {
+                    console.warn(
+                        'Select2: The `select2("data")` method can no longer set selected values in 4.0.0, ' +
+                        'consider using the `.val()` method instead.'
+                    );
+                }
+
                 if (arguments.length > 1) {
                     triggerChange = arguments[1];
                 }
@@ -3066,7 +3234,7 @@ the specific language governing permissions and limitations under the Apache Lic
             });
             this.setVal(val);
         },
-        
+
         createChoice: function (data) {
             var enableChoice = !data.locked,
                 enabledItem = $(
@@ -3442,6 +3610,7 @@ the specific language governing permissions and limitations under the Apache Lic
 
     // plugin defaults, accessible to users
     $.fn.select2.defaults = {
+        debug: false,
         width: "copy",
         loadMorePadding: 0,
         closeOnSelect: true,