Respect parent lookup regex value

Opening this preemptively because I need some guidance and discussion. 

I tried using the core's <a href="https://github.com/tomchristie/django-rest-framework/blob/4e001dbb7ac0bc13d6d5fbb4524e905184610aa2/rest_framework/routers.py#L216">get_lookup_regex</a> method, but ran into a couple of snags. The most important one is that it's not really hard to respect the parent's `lookup_value_regex`, but `lookup_value_regex()` also takes into account the viewsets `lookup_field` which we don't set anywhere in the current implementation. I am fighting with it right now, but not even sure if it's possible to do it remotely pretty. What do you think?