public static function customButton($title, $url, $attributes = array())
    {
        // configuration
        $config = self::config();

        // no configuration file
        if (!$config) {
            return null;
        }

        $attributes = Helpers::addClass($attributes, "clef");

        return '<a href="https://clef.io/iframes/qr?app_id='.$config['app_id'].'&amp;redirect_url='.$url.'" '.\HTML::attributes($attributes).'>'.$title.'</a>';
    }