Hls.js: Add WebVTT container parsing

[jlacivita]

hls.js should support at least parsing out the VTT tracks from the EXT-X-MEDIA tag, and exposing the URIs.

Potentially, individual VTT "fragments" should be parsed into VTTCues for the video element.

[jlacivita]

I'm working on this:
https://github.com/jlacivita/hls.js/tree/vtt

but 708 is higher priority for me right now. Any help or comments welcome.

I did the basic refactor to playlist-loader to support parsing levels and text-tracks separately. same technique can eventually be used for alternate audio.

[azaslavsky]

What is the status of this change?

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.