code: function(name){
          for (var i = 0, len = _pluginList.length ; i < len ; i++){
              var plugin = _pluginList[i];
              if (name === plugin.name) {
                  return true;
              }
          }
          return false;
      }
docs: weather the plugin is exist in plugin's list or not
  @param  {String}  name the plugin's name
  @return {Boolean}