Allow passing of kwargs to django queryset explains in analyze_queryset

[pmaresca]

In some instances Django's explain() can accept additional keyword args. At the moment analyze_queryset() doesn't pass any kwargs on Django explain() calls. So if you use this utility on any Django querysets related to a Postgres db, you only get EXPLAIN instead of EXPLAIN ANALYZE. Passing these args when available and supported would be rather convenient and make analyze_queryset actually analyze on Django QuerySet objects.