Add ability to make some OPTION tags selected by default as per provided data

Just set the 'selected' property to true on the data item
Works also for optgroup's options.

Also use jQuery tag constructor and attribute setter to avoid HTML filtering issues

js/bootstrap-multiselect.js

@@ -967,30 +967,63 @@
 
         /**
          * The provided data will be used to build the dropdown.
-         * 
-         * @param {Array} dataprovider
+         *
+         * @param {Array} dataprovider Array of OPTION or OPTIONGROUP models.
+         * A simple OPTION tag is represented as:
+         * ```js
+         * {
+         *  value: "option value"
+         *  label: "option label" // The `value` is used if this is falsy
+         *  selected: false // If `true`, mark the OPTION tag as selected
+         * }
+         * ```
+         *
+         * An OPTIONGROUP tag is represented as:
+         * ```js
+         * {
+         *  label: "optiongroup label"
+         *  children: [
+         *      {
+         *        // same as option model
+         *      }
+         *  ]
+         * }
+         * ```
          */
         dataprovider: function(dataprovider) {
             var optionDOM = "";
             var groupCounter = 0;
+            var tags = $(''); // create empty jQuery array
 
             $.each(dataprovider, function (index, option) {
-                if ($.isArray(option.children)) {
+                var tag;
+                if ($.isArray(option.children)) { // create optiongroup tag
                     groupCounter++;
-                    optionDOM += '<optgroup label="' + (option.title || 'Group ' + groupCounter) + '">';
-
-                    forEach(option.children, function(subOption) {
-                        optionDOM += '<option value="' + subOption.value + '">' + (subOption.label || subOption.value) + '</option>';
+                    tag = $('<optgroup/>').attr({
+                        label: option.title || 'Group ' + groupCounter
+                    });
+                    forEach(option.children, function(subOption) { // add children option tags
+                        tag.append($('<option/>').attr({
+                            value: subOption.value,
+                            label: subOption.label || subOption.value,
+                            selected: !!subOption.selected
+                        }));
                     });
 
                     optionDOM += '</optgroup>';
                 }
-                else {
-                    optionDOM += '<option value="' + option.value + '">' + (option.label || option.value) + '</option>';
+                else { // create option tag
+                    tag = $('<option/>').attr({
+                        value: option.value,
+                        label: option.label || option.value,
+                        selected: !!option.selected
+                    });
                 }
+
+                tags = tags.add(tag);
             });
             
-            this.$select.html(optionDOM);
+            this.$select.empty().append(tags);
             this.rebuild();
         },