Add an new item (key, value) to the current history.

def add(self, key, value,
            description='',
            history_max_size=None):
        """Add an new item (key, value) to the current history."""
        if key not in self.stats_history:
            self.stats_history[key] = GlancesAttribute(key,
                                                       description=description,
                                                       history_max_size=history_max_size)
        self.stats_history[key].value = value