Add -help

[basxto]

Add -help/-?/--help (all three) for command line options, which explains what commands can be used and what arguments they expect.

Partly resolves #76 and #77 since I just didn’t see them.

It’s probably best/easiest to look for a library that handles command line options, those are usually capable of generating help automatically.

Edit: https://picocli.info/ might be an option.