Add a subcommand to generate a completion script

[corneliusweig]

Cobra has a builtin generator to create completion scripts for bash and zsh. Many projects make use of this generator, among others sigs.k8s.io/krew, k8s.io/minikube, or GoogleContainerTools/skaffold. This is usually used as source <(berglas completion zsh) in your shell, which then offers an improved CLI experience -- especially for new and casual users.

Is there a particular reason why berglas does not want that?

[sethvargo]

I think it was just an oversight. I'll try to get this into the next version.

[corneliusweig]

If you like I can get my hands dirty too.

[sethvargo]

7552f02