dub zsh completion

[Johannes Loher via Digitalmars-d-announce]

I created a zsh completion script for dub. It is not perfect, but 
it does many things well already. You can find it here: 
https://github.com/ghost91-/dub-zsh-completion.

I have seen that bash and fish completion scripts are included in 
the dub github repo. If people are interested in this, I could 
create a pull request to also add the zsh completion script.

I hope this helps some people :)