exits with success status when no COMMAND argument is given

[Akuli]

from manly.py:

    if not len(args.command):
        print_err(
            "manly: missing COMMAND\nTry 'manly --help' for more information.",
        )
        sys.exit(0)

many programs use 2 for invalid usage, argparse does it too

[Summertime]

man returns 1 for missing arg, which is probably what we should follow

[Summertime]

Fixed in 4a6ad1f, need to follow up with making tests, ! manly or manly ; [[ $? -eq 1 ]] should be best