Add completion for fish shell

[hellow554]

I'd like to see a completion for the fish shell

[kytrinyx]

Agreed, that would be lovely.
See #666 for reference to the Cobra documentation for completion scripts, and #759 for the details about how this was done for Bash and Zsh

[olingern]

@kytrinyx I think this can be closed a la #763

[kytrinyx]

Agreed! Thanks so much :-)