youtube-dl output plugin

Works so far
There are some missing arguments, but I'm not sure if Flexget should support absolutely everything. If somebody needs that much fine-control, they're better off making their own watch folder for youtube-dl or something.

Passes PEP8
PyLint sees only minor issues, rated at 8.78/10
No unit tests because I don't know how :)
Closes [Trac ticket 1814](http://flexget.com/ticket/1814) by paranoidi