Add support for zsh-completion.

[xu-cheng]

Currently youtube-dl supports both bash and fish auto completion. It would be nice to see it supporting zsh too.

[jaimeMF]

I don't use zsh myself (I don't know about other contributors), could you link to some documentation about its completion system?
If you know a bit of python (not too much is needed) and have time, you can look into the bash and fish completion generators( bash-completion.py, bash-completion.in, fish-completion.py and fish-completion.in) and modify them to output the zsh completion script. We'll gladly review a pull request.

[xu-cheng]

Well, although I'm a zsh user, I know a lot about python but very few on zsh completion system. I see whether I can give it a try.