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

package.json

@@ -19,6 +19,7 @@
     "application-config-path": "^0.1.0",
     "bitfield": "^1.0.2",
     "chromecasts": "^1.8.0",
+    "concat-stream": "^1.5.1",
     "create-torrent": "^3.22.1",
     "deep-equal": "^1.0.1",
     "dlnacasts": "^0.0.3",
@@ -34,6 +35,7 @@
     "prettier-bytes": "^1.0.1",
     "rimraf": "^2.5.2",
     "simple-get": "^2.0.0",
+    "srt-to-vtt": "^1.0.3",
     "upload-element": "^1.0.1",
     "virtual-dom": "^2.1.1",
     "webtorrent": "^0.90.0",