code: function (config, key, defaultValue) {
      if (false !== defaultValue) {
          defaultValue = true;
      }
      var result = defaultValue;
      if (typeof config === 'object' && typeof key === 'string' && config[key] && 'boolean' === typeof config[key].enable) {
          result = config[key].enable;
      }
      debug('middleware:', key, 'enable', result);
      return result;
  }
docs: enable middleware
  @param config {object}
  @param key {string}
  @param defaultValue {boolean} default value for config[key].enable
  @returns {boolean}
  default return true