/** Load a module plugin **/
 public function plugin($plugin)
 {
     if (isset($this->_ci_plugins[$plugin])) {
         return;
     }
     list($path, $_plugin) = Modules::find($plugin . '_pi', $this->_module, 'plugins/');
     if ($path === FALSE) {
         return parent::plugin($plugin);
     }
     Modules::load_file($_plugin, $path);
     $this->_ci_plugins[$plugin] = TRUE;
 }