Youtube plugin

I'm not sure why there are 15 commits here, it most likely is because I didn't rebase ever. I don't quite like making new branches for the sole purpose of pushing two commits, but here we are.

Anyway whilst I usually have access to youtube-dl on computers I use, I thought I might as well make a web module too.

You will need to run `git submodule init` and `git submodule update` or just `git clone https://github.com/rg3/youtube-dl.git` in the main folder.