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.js

@@ -1030,6 +1030,23 @@ class Plyr {
     return this.media === document.pictureInPictureElement;
   }
 
+  /**
+   * Sets the preview thubmnails for the current source
+   */
+  setPreviewThumbnails(thumbnailSource) {
+    if (this.previewThumbnails && this.previewThumbnails.loaded) {
+      this.previewThumbnails.destroy();
+      this.previewThumbnails = null;
+    }
+
+    Object.assign(this.config.previewThumbnails, thumbnailSource);
+
+    // Create new instance if it is still enabled
+    if (this.config.previewThumbnails.enabled) {
+      this.previewThumbnails = new PreviewThumbnails(this);
+    }
+  }
+
   /**
    * Trigger the airplay dialog
    * TODO: update player with state, support, enabled