Added preview thumbnails setter & fixed cannot set property 'hover' of null (#2256)

* added iPad to isIos

* elements.controls can be null

* elements.controls can be null

* added previewThumbnails setter

* fixed readme formatting

* previewThumbnails setter to method

src/js/plyr.d.ts

@@ -211,6 +211,11 @@ declare class Plyr {
    */
   airplay(): void;
 
+  /**
+   * Sets the preview thubmnails for the current source.
+   */
+  setPreviewThumbnails(source: Plyr.PreviewThumbnailsOptions): void;
+
   /**
    * Toggle the controls (video only). Takes optional truthy value to force it on/off.
    */
@@ -619,6 +624,11 @@ declare namespace Plyr {
      * Booleans are converted to HTML5 value-less attributes.
      */
     tracks?: Track[];
+
+    /**
+     * Enable or disable preview thumbnails for current source
+     */
+    previewThumbnails?: Plyr.PreviewThumbnailsOptions;
   }
 
   interface Source {