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.
DOM elements can have event handlers added to them via `foo.onchange = function() {};`. This allows that mechanism.