def _add_user_source(self):
        """Add the configuration options from the YAML file in the
        user's configuration directory (given by `config_dir`) if it
        exists.
        """
        filename = self.user_config_path()
        if os.path.isfile(filename):
            self.add(ConfigSource(load_yaml(filename) or {}, filename))
Add the configuration options from the YAML file in the
        user's configuration directory (given by `config_dir`) if it
        exists.