Add shell completion subcommand

Implements feature request #164 I added all shell completions that `clap_complete` supports out of the box. I didn't test them apart from checking that the script generation does not panic. For bash, I also checked that I get some completions but I didn't check if completions for all parameters are there. This PR could be expanded upon with compile time generation of completions and adding those into the packaging but just having the command should be enough for a first version.