resolve [issue 171] : offline compressor to compress 'compress tags' if those are not in child template but in parent template.

Offline compressor will create cache files for this type of template inheritance.

Parent template:

{% compress css %}
    {% block css %}
        ...
    {% endblock %}
{% endcompress %}

Child template:

{% block css %}
    {{ block.super }}
    <more CSS links>
{% endblock %}