[Twig] Add references for blocks in embed tag

[Haehnchen]

http://twig.sensiolabs.org/doc/tags/embed.html

{% embed "teasers_skeleton.twig" %}
    {% block left_teaser %}
        Some content for the left teaser box
    {% endblock %}
{% endembed %}

Features:

• block completion
• block navigation [Twig] Blocks overriden in a {% embed %} tag should allow navigating to the parent, as with {% extends %} #513

[simshaun]

+1