webvtt format support

[tamplin]

Netflix supplied new "webvtt-lssdh-ios" format，such as Chinese.

[isaacbernat]

Hi, do you have a sample and/or a procedure on how to get the new format?

[isaacbernat]

@qcharoo if you can't provide an example I will close the issue

[isaacbernat]

Ok, @qcharoo since you don't give me more information I'll close the issue

[isaacbernat]

@qcharoo I added support for .vtt from some samples I found around. Feel free to try it out and star the project.