Fix zsh autocompletion

[amenzhinsky]

Fixes: #105

Probably not the nicest fix, but makes zsh autocompletion work.

Unfortunately there's no single styleguide for cli apps to follow, but docker, systemctl and kubectl declare available option values in parentheses.

[bufdev]

Can you file an issue to fix cobra instead? There's actually a bunch more places this needs to be changed, and is a bit more involved, as most of these are auto-created (we'd have to change from [ to ( in all of them to be consistent). We can do it depending on Cobra's response, but I would prefer to print out [.

[bufdev]

This should be fixed in Cobra instead, or if the fix is already present and we are misusing Cobra, we should fix that. See spf13/cobra#1129