Add auto completion via argcomplete

[willemt]

Because cliar uses argparse, auto completion works right out of the box when integrating with argcomplete

[moigagoo]

@willemt thank you so much for the contribution!

May I ask you to add a few words about autocompletion to the docs? Like, what is autocompleted, how to enable the completions, and what the limitations are.

Also, I wonder, is there a way to test that functionality? 🤔