Implement bash tab completion

[timbod7]

Useability would be much improved with a bash completion script that knows how to complete:

• subcommands
• releases
• deploys
• endpoints

[timbod7]

This is mostly a case of writing a bash script as described here:

https://debian-administration.org/article/316/An_introduction_to_bash_completion_part_1

Though it would be easiest to implement if camus2 were extended with a few helper subcommands:

c2 show-endpoint-names
c2 show-deploy-names

[timbod7]

Or, probably better, switch to parsing args with optparse-applicative, which apparently has bash completion built in:

https://github.com/pcapriotti/optparse-applicative/wiki/Bash-Completion