add data shortcut for attribute expression

A new feature being introduced in HTML 5 is the addition of custom data attributes.

As example :

```html
<div class="user" data-name="Akarzim" data-city="Lille">
  <b>Akarzim says:</b> <span>Hello, how are you?</span>
</div>
```

With this new expression, as a shortcut of `XPath.attr(:"data-name")` now we can use `XPath.data(:name)`