Allow specifying args in a file.

Helpful for when command lines are otherwise too long.
Since this is reused, how about making it a separate decorator which does this part:

    paths = [path for path_glob in path_globs for path in glob.glob(path_glob)]
    if argfile:
      paths.extend(argfile.read().splitlines())
      argfile.close()

Or some variant on it based on how you wanted to handle errors, etc