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.

This is **not** ready to merge yet.