code: function addExecutor(name, executor) {
    if (this._executors[name]) throw new Error('executor "' + name + '" is already defined');
    this._executors[name] = executor;
  }
docs: add external executor to JSONScript interpreter
  @this  JSONScript
  @param {String}          name     executor name to use in $exec keyword
  @param {Object|Function} executor executor object or function