Add bash completion for array element addition and removal

[sim642]

This allows bash completing things like --set witness.yaml.entry-types[+] to offer a list of valid things to add or remove (with [-]) from the list.

[michael-schwarz]

I personally don't have argument completion set up, so I didn't test it. However, it seems like an innocent local thing we should probably just merge.

[arkocal]

Works fine on my system.