Allow configuration of DefaultRouter's root view renderer classes

[marcgibbons]

Currently, we can set the schema_renderer classes as kwargs on the DefaultRouter, but can't override the renderer classes used on the root view which comes from the settings.

https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/routers.py#L292

It would be desirable to replace BrowsableAPI renderer with the SwaggerUI html renderer in Django REST Swagger.

Referencing issue:
marcgibbons/django-rest-swagger#479 (comment)

[tomchristie]

Thanks Marc. Getting back onto the project after DjangoCon - apologies for the delay.

[marcgibbons]

100% thanks for all the hard work!