Support native captions in html5 tech.

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.

This is the beginning of step 1 for the [revamp](https://github.com/videojs/video.js/issues/1655) of text tracks.

* [x] Native tracks, if available
* [x] Re-use non-native otherwise
* [x] Chrome Support
* [x] Firefox support
* [x] iPad Support
* [x] Android support
  * [x] Chrome for Android -- works like desktop chrome
  * [x] Native Android Browser
* [x] IE Support
  * [x] IE11
  * [x] IE10
  * [x] IE9
  * [x] IE8 
* [x] Tests
If we're able to fully shim tracks, showTextTrack would go away and instead we'd be changing mode to 'showing' on the track objects themselves.

In order to do that, our mock text track objects would need defined properties that look just like native tracks and can respond to changes (Object.defineProperty including its IE8 limitations). Either that or we would always wrap native text tracks in a videojs-ish API (e.g. `textTrack.mode('showing')`), so the same API could be used for both. I'm guessing the first option is the preferred direction, including figuring out how to deal with the IE8 issue.

Would all this be part of the vtt.js research?