Added support for generic messages via data-msg attribute

This extends the `customDataMessage` method to add support for setting a generic message  on a field without having to use the `title` attribute method:

```html
<input type="email" class="required" data-msg="Please enter something to continue.">
```

Method specific messages still have precedence which means that this also works:

```html
<input type="email" class="required" data-msg="Enter something" data-msg-email="Enter a valid email">