[Attributes] data-class implementation

This PR implements data-class functionality.

data-class expects a valid JSON object as a value: keys are the class names to add; the value is used as a condition, a truthy value causes the class to be added to the target element. 

Example usage:

```html
<div data-class='{"myClass": true}'></div>
<div data-class='{"anotherClass": "{{someScopeFunction()}}"}'></div>
<div class="hello" data-class='{"hello": false, "salute": true}'></div>
```