Additional report parameter: list_filter_queryset.

Now we can query Foreign Key fields like User.objects.filter(is_superuser=True).

Usage: in main reports.py file add additional parameter:

list_filter_queryset = { 'user': {'is_superuser': True}, 'author': {'pk__in': [9,10]} }
Merge pull request #36 from jelenak/master

Additional report parameter: list_filter_queryset.