Set the source type for youtube videos

[arthurian]

This PR fixes an issue with youtube videos not loading due to incorrect playback tech being used by videojs. It looks like the videojs youtube tech is expecting the source type to be video/youtube, otherwise it won't work.

Notes:

• Detects youtube videos that have URLs like https://youtu.be/ID or https://www.youtube.com/watch?v=ID and sets the source_type to video/youtube, otherwise defaults to video/mp4.
• This will require a subsequent update to hxighlighter_video_vjs.js to fix a related issue with the videojs youtube tech. This change can be merged before that, since it shouldn't negatively affect any existing functionality. See also: https://github.com/lduarte1991/Hxighlighter/pull/24

@lduarte1991

[lduarte1991]

@arthurian Can definitely merge this asap. just let me know that it doesn't clash with any of the other you've submitted and I'll do it now.

[arthurian]

@lduarte1991 There shouldn't be any conflict with the other PRs, so feel free to merge when ready.