Captions

This is the new all encompassing captions PR.
It's still have some issues, like `cuechange` firing unconditionally and some issues with the captions displaying but there won't be much architectural changes to this.

I'd like at some point to take all the TextTrack stuff out of videojs and include it as a dependency similar to vttjs.
It wasn't playing nicely. It didn't want to load the text tracks that were there natively and let us change their mode.
Firefox is theoretically supported but I didn't have time at then to figure out with firefox wasn't working like it should.