Parameter `--osxnotify` is hard to negate

The only way I know to pass `bool(False)` with the current implementation is to pass in an empty string: `osxnotify=''` -- _argparse_ does no implicit cast from `str` to `bool` other than the default Python truth test for empty sequences. So one must either write a `str2bool` function or add a `--no-osxnotify` option, using _argparse.add_argument_'s `action='store_true'` and `action='store_false'`.

Or maybe drop the option altogether and let it be disabled with `-p no:osxnotify` once #9 is fixed.