provide completion for zsh

[koiuo]

Writing zsh completion manually is too time consuming (at least for me), so in 1.8.0 it was removed from the app. Instead randrctl setup completion command was implement which generates bash completion from argparse model using argcomplete. zsh users can still use _bash_completion in zsh.

Native zsh completion, however, would be highly desired (at least I miss it a lot). Volunteers wanted to develop zsh completion manually or to suggest a library that would generate completion from argparse model, like argcomplete does for bash.