Don't auto-add the textTrackDisplay component.
Add a 'featuresNativeTrack'. Currently hardcoded to false for everything
except the html5 tech.
Add textTracks and addTextTrack methods to html5 tech.
Proxy player level methods to tech if we're using native tracks.
Fix up track menu items to work with spec tracks or vjs tracks.

Also, if text tracks are old and use numerical mode values, use custom
tracks.
If an html tech is using custom tracks, remove track elements so we
don't accidentally show both custom and native captions.
Clean up showTextTrack slightly.

textTracks is completely inside of techs but needs to be called manually
because techGet requires the tech to be ready.
addTextTrack, unfortunately, currently forks. If when called, it doesn't
have a tech, it assumes that it's a custom implementation and does the
same work that MediaTechController#addTextTrack does.
Don't create a flash getter for textTracks.

Load up textTrackDisplay synchronously but have it do no work until the
player is ready (see the previous commit).

Add a check that delegates to MediaTechController's methods if we're in
Html5 but we are not using native tracks.

MenuButton's new 'update' method conflicted with volumeMenuButton's
update method which was used to update volume levels.

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.

vtt.js is now loaded at the first text track creation so don't load it in the media tech controller as well.

vtt.js handles captions styling so don't bother with text track styles in video-js.less anymore.

Since vtt.js is parsing captions now, we don't need a unit test for captions parsing.

Put the option to override the URL to vtt.js somewhere that it can be easily accessed.