Added use statement for embed tag

My proposal for #1560.

It makes it possible to use the `print` block form the main template within the `embed` area in the following example:

    {# layout.html #}
    <h1>Website</h1>
    {% block content '' %}

    {# panel.html #}
    <div class="panel">{% block content '' %}</div>

    {# index.html #}
    {% extends 'layout.html.twig' %}

    {% block print %}
        {{ x }}
    {% endblock %}

    {% block content %}
        {% embed 'panel.html.twig' use print %}
            {% block content %}
                {% for x in ['a', 'b', 'c'] %} {{ block('print') }} {% endfor %}
            {% endblock %}
        {% endembed %}
    {% endblock %}

What has been added is the `use` keyword for the `embed` tag. Maybe the implementation is not optimal, since I just skimmed over the source to see how stuff is usually done, but I can improve it if you're interested in including this.