Create props objects for blocks from data-* attributes

For this markup
```slim
@@block data-property='foo' data-another-property='bar'
  @element data-property='baz'
````

these properties will be created
```coffeescript
@block.props.property # 'foo'
@block.props.anotherProperty # 'bar'
@element.props.property # 'baz'
```