public function __construct($extra_config = array())
 {
     parent::__construct();
     if (!empty($extra_config) && is_array($extra_config)) {
         $merged_configuration = $extra_config + $this->configuration;
         if (isset($extra_config['classes']) && isset($this->configuration['classes'])) {
             $merged_configuration['classes'] = $extra_config['classes'] + $this->configuration['classes'];
         }
         $this->configuration = $merged_configuration;
     }
 }