Add ZSH autocompletion support

[johnsudaar]

[netgusto]

I believe it's supported out of the box by our cli handler; I'll have to confirm.

[netgusto]

It is supported out of the box.

Will be triggered by --generate-bash-completion (standard flag for this)

Will support command scopes; and maybe contextual data listing if available (list of available maps, list of available agents, etc.)

[xtuc]

Seems not supported right now. PR welcomed :)