Add the support detection of active menu items

[execut]

I think the item can have two methods for detecting active menu items from the template:
is_active() - for detect a active item.
is_has_active_children() - for detection of what item has active children.

Usage example:

{% load multimenus %}

{% for item in items %}
<li class="list-group-item{% if item.is_active or item.is_has_active_children %} active{% endif %}">
    ...
</li>
{% endfor %}