add is_active stringfilter

[vad]

With {% ifswitch %} is not possibile to add other conditions, this is a nice complement that can be used if you need to do something like:

    {% if request|is_active:"switchname" and not user.is_anonymous %}
      <my>html</my>
    {% else %}
       <my>other html</my>
    {% endif %}

In this way you don't write <my>other html</my> twice.