Added string overload for 'data' to return appropriate type in 'royalslider'.

royalslider/royalslider-tests.ts

@@ -100,7 +100,7 @@ jQuery(document).ready(function () {
 // Another example: $(".royalSlider").royalSlider('goTo', 3);
 // But it's recommended to get instance once if you have many calls:
 
-var slider: RoyalSlider.RoyalSlider = $(".royalSlider").royalSlider().data('royalSlider');
+var slider = $(".royalSlider").royalSlider().data('royalSlider');
 
 slider.goTo(3); // go to slide with id
 slider.next();  // next slide

royalslider/royalslider.d.ts

@@ -493,4 +493,6 @@ interface JQuery {
     * @param options The options
     */
     royalSlider(options?: RoyalSlider.RoyalSliderOptions): JQuery;
+
+    data(key: "royalSlider"): RoyalSlider.RoyalSlider;
 }