docs: def arg(*args, **kwargs):
      """Return an attrib() that can be fed as a command-line argument.
  
      This function is a wrapper for an attr.attrib to create a corresponding
      command line argument for it. Use it with the same arguments as argparse's
      add_argument().
  
      Example:
  
      >>> @attrs
      ... class MyFeature(Feature):
      ...     my_number = arg('-n', '--number', default=3)
      ...     def run(self):
      ...         print('Your number:', self.my_number)
  
      Now you could run it like `firefed myfeature --number 5`.
      """
      metadata = {'arg_params': (args, kwargs)}
      return attrib(default=arg_default(*args, **kwargs), metadata=metadata)