Add subtitles for videos

[tobiasrohloff]

When subtitles are available for a video, the user should be able to display them. I read that the video player on iOS supports it, but please validate it. Also it would be interesting if tvOS supports it.

But first, the new API has to deliver links to the subtitle files. I already requested that. We use the webVTT format.

[tobiasrohloff]

The subtitle vtt_urls are now available for the video resource in the subtitles hash. Thus, this feature can be implemented know.