Provide shell autocompletion

A feature that can be useful is command autocompletion, where you can hit `TAB` and the shell can provide suggestions:

[Completely - Bash Completions Generator](https://github.com/DannyBen/completely#completely---bash-completions-generator)

[Bash TAB completion tutorial](https://www.gnu.org/software/gnuastro/manual/html_node/Bash-TAB-completion-tutorial.html)

