title: Parsley custom dynamic error message
body: <p>Parsley (2.8)
  I have a custom validator</p>
  
  <pre><code>window.Parsley.addValidator('password', {
      validateString: function(value, req, elem) {
          var error = validatePassword(value);
          return error === '';
      },
      messages: {
          en: 'Want my custom message based on input value'
      }
  });
  </code></pre>
  
  <p><code>validatePassword(value)</code> return error message according to value, e.g. special character is missing. So how I can access value or input element inside error message. </p>