fix help

[orsinium]

Close #36

[webknjaz]

This probably needs a test

[orsinium]

Too hard to test, flake8 raises sys.exit after showing help. Contributions are welcome, as always. And contributions with tests are welcome twice :)

[webknjaz]

Well, maybe call in the subprocess and check if something is in the stdout... Also, sys.exit() raises SystemExit internally. So I guess it's

pytest.raises(SystemExit, match=...):
    ...