Add a new value for a key.

        This differs from __setitem__ in adding a new value instead of updating
        the list of values, thus avoiding the need to fetch the previous list of
        values.

def add(self, key, value):
        """Add a new value for a key.

        This differs from __setitem__ in adding a new value instead of updating
        the list of values, thus avoiding the need to fetch the previous list of
        values.
        """
        self.configfile.add(self.name, key, value)