Install proposal app

Let contributors to add new categories or videos (as well as edit existing categories and videos) through pull requests.

Few notes:
* Video proposals reside inside the __drafts__ directory
* Supported formats are yaml, json and xml
* To review and add proposed videos run the __review__ command
I guess we should fail as late as possible to enable fixing most of the problems (now we fail asap if we find errors).
However, this is not important