zsh: rework ZSH completion

[vincentbernat]

The completion for ZSH is upgraded to the new ZSH completion system. To work, the file should be placed somewhere in ZSH fpath to be autoloaded when needed and named _sub.

There are other embedded misc changes:

• $CURRENT indicate which word is being autocompleted.
• In ZSH, automatic word splitting is disabled in completion. There is no need to quote words.

The completion also handles plugins that are able to complete args. However, most plugins don't and will complete with a subcommand instead. I don't think there is an universal solution for this.

I am no ZSH wizard. Take this commit with care (works for me).

[mislav]

Looks good!

[markstos]

I'm not a ZSH user, but given that @mislav seems to have peer-reviewed this, I recommend merging this.