Add enable_data_attributes option that allows using HTML5 data +  attributes as control attributes instead or in addition to XML + namespace attributes

If set, data attributes can be used instead of namespace attributes, e.g. "data-tal-content" instead of "tal:content"

Sample:

``` html
<html>
  <body>
    <div foo="bar" data-tal-content="'Hello world!'" />
  </body>
</html>
```
