Use vtt.js for track element emulation
Remove video.js's track element emulation and pull in vtt.js to do that work instead. Add an option to force vtt.js-based captions to be used even if native caption support is present. Currently, vtt.js is loaded when the first TextTrack component is created. If vtt.js is not available when it's time to begin parsing cues, poll until it finishes downloading.
Finish moving lazy load
vtt.js is now loaded at the first text track creation so don't load it in the media tech controller as well.
Fix styling.
vtt.js handles captions styling so don't bother with text track styles in video-js.less anymore.
Remove obsolete cue parsing test
Since vtt.js is parsing captions now, we don't need a unit test for captions parsing.
Allow overriding vtt.js URL
Put the option to override the URL to vtt.js somewhere that it can be easily accessed.
Merge pull request #3 from dmlap/native-captions

vtt.js captions