code: function (fn) {
      if (!fn || !_.isFunction(fn) || fn.length !== 1) {
        var err = new Error('Please pass a valid function!');
        debug(err);
        throw err;
      }
  
      this.validatorExtension = fn;
  
      return this;
    }
docs: Extends the validator instance.
  @param   {Function} fn the function to be injected
  @returns {Domain}      to be able to chain...