Add a new html_attributes to work with angular.js

Angular JS require to add a new attribute ng-app to the html tag.

This new block allow us to add it, for example :

{% block html_attributes %}ng-app="PrestaCMS.Page"{% endblock %}
Add class="no-js" directly in the block
Merge pull request #1639 from nicolas-bastien/patch-1

Add a new html_attributes to work with angular.js