Parameter `--osxnotify` is hard to negate

[wcooley]

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.