Add subtitle support (via drag-n-drop) (#361)

* issue template

* cleanup closePlayer() and stopServer()

* Add subtitle support (via drag-n-drop)

Drag and drop a subtitles file (.srt or .vtt) onto the player (or the
app icon on OS X) to add subtitles to the currently playing video.

For #281

* add multiple subtitles structure

* add open subtitle dialog from cc player controls

renderer/state.js

@@ -70,7 +70,11 @@ function getDefaultPlayState () {
     isPaused: true,
     isStalled: false,
     lastTimeUpdate: 0, /* Unix time in ms */
-    mouseStationarySince: 0 /* Unix time in ms */
+    mouseStationarySince: 0, /* Unix time in ms */
+    subtitles: {
+      tracks: [], /* subtitles file (Buffer) */
+      enabled: false
+    }
   }
 }