Adds admin list filter for polymorphic child models.

This is an enhancement for the admin. Adding this to a PolymorphicParentModelAdmin's .list_filters allows users of the admin to filter the change list by the child models.

I use this basically every time I use django_polymorphic so I thought I'd see if you felt it would be worth including. That, or someone will point out a better way to accomplish this.

Thanks again for maintaining this project.