Enable Bash completions

[bswck]

No description provided.

[pawamoy]

@bswck I went with a CLI option (hidden from the generated help) to avoid the CLI erroring out because no arguments were passed. This initial draft works well!

TODO:

• the TODO comments
• add a --completion CLI option that renders the completions.bash script (binary and spaced_names variables) and print it on stdout, so that users can duty --completion > /path/to/completions/duty.completions.bash.