Add zsh completions

[jez]

There is already a bash completion script, and I use zsh, so that's motivation enough.

The documentation here looks quite promising: https://github.com/zsh-users/zsh-completions/blob/master/zsh-completions-howto.org

[stylemistake]

You can use this as a template: https://github.com/gruntjs/grunt-cli/blob/master/completion/zsh