add preview thumbnails

Signed-off-by: Jesús <heckyel@hyperbola.info>

youtube/static/js/plyr-start.js

@@ -116,6 +116,10 @@
         }
       },
     },
+    previewThumbnails: {
+        enabled: true,
+        src: [storyboard_url],
+    },
     settings: ['captions', 'quality', 'speed', 'loop'],
   });
 }());