Added 'optgroups' option property in 'selectize'.

selectize/selectize.d.ts

@@ -183,6 +183,13 @@ declare module Selectize {
          */
         valueField?: string;
 
+        /**
+         * Option groups that options will be bucketed into.
+         * If your element is a <select> with <optgroup>s this property gets populated automatically.
+         * Make sure each object in the array has a property named whatever "optgroupValueField" is set to.
+         */
+        optgroups?: U[];
+
         /**
          * The name of the option group property that serves as its unique identifier.
          *