code: function listen(name, callback) {
      if (!showdown.helper.isString(name)) {
        throw Error('Invalid argument in converter.listen() method: name must be a string, but ' + typeof name + ' given');
      }
  
      if (typeof callback !== 'function') {
        throw Error('Invalid argument in converter.listen() method: callback must be a function, but ' + typeof callback + ' given');
      }
  
      if (!listeners.hasOwnProperty(name)) {
        listeners[name] = [];
      }
      listeners[name].push(callback);
    }
docs: Listen to an event
  @param {string} name
  @param {function} callback