title: Knockout Validation Plugin Custom Error Message
body: <p>Based on the following, how exactly do I setup the callback to display a custom error message instead of the default message?</p>
  
  <pre><code>ko.validation.rules['exampleAsync'] = {
      async: true, // the flag that says "Hey I'm Async!"
      validator: function (val, otherVal, callBack) { // yes, you get a 'callback'
  
          /* some logic here */
  
          // hand my result back to the callback
          callback( /* true or false */ );
          // or if you want to specify a specific message
          callback( /* { isValid: true, message: "Lorem Ipsum" } */ );
      },
      message: 'My default invalid message'
  };
  </code></pre>