CLI: show usage when called with --help

[x1B]

The CLI usage is currently described in the README, but should also be shown when called with --help

[qacwnfq]

I'm thinking about doing a PR for this.
It's probably not worth adding another dependency to some command-line parser.
Instead I think it would be cool to grab the usage directly from the README and output it.
Also usage could be shown, when there are unrecognized options passed.

[x1B]

@qacwnfq I like this idea, both grabbing output from the readme, and showing the help when an unrecognized option was passed.