Add support for using youtube-dl to playback files from various streaming sites

[cnelson]

If we have youtube-dl installed, use it to fetch content for us for the sites it supports.

[cnelson]

Implementation Sketch:

I want to be able to do things like:

$ airplay https://www.youtube.com/watch?v=lDba6HBbmqE

Once #2 is completed, and we have ffmpeg in our pipeline we can use youtube-dl to extract video URLs for the streaming services it supports. It's very simple:

>>> import youtube_dl
>>> y = youtube_dl.YoutubeDL()
>>> y.extract_info('https://www.youtube.com/watch?v=lDba6HBbmqE', download=False)

Then we can pass those video formats to ffmpeg for transcode (if needed!) just like any other direct video link.

TL;DR