public function get_config($section, $key = '', $default = null) {
        if ($section === '' || !isset($this->data[$section])) {
            return null;
        }
        if ($key === '') {
            return $this->data[$section];
        }
        if (!isset($this->data[$section][$key])) {
            return $default;
        }
        return $this->data[$section][$key];
    }