Support clip subtitles

For now always chooses first subtitle from array until #78, #84, #500
and #501 are implemented.

This kinda fixes #431 as one can now implement their own VTT parser
with:

```js
flowplayer.conf.subtitleParser: function(txt) {
  return [{title: '', startTime: 100, endTime: 120, text: ''}];
};
```