Add shell completion

[wwerner]

Add shell completion script generation commands for

• bash
• zsh

References:

• https://github.com/spf13/cobra/blob/master/bash_completions.md
• https://debian-administration.org/article/316/An_introduction_to_bash_completion_part_1
• https://kubernetes.io/docs/tasks/tools/install-kubectl/#enabling-shell-autocompletion