Required options cannot be in config files

[jyn514]

Argparse won't see them in the second call to parse_known_args and will throw an error.

I thought a workaround would be to prepend new_args to args, but then that doesn't play well with append: if you have group = a in some config and --group b on the command line, you'd expect b to overwrite a, not to be appended.

Now that I think about this some more, I don't think the current system interacts well with mutually_exclusive_args at all ...