Add support for creating HTML forms for the browsable API for "extra actions" for routing.

For example using the example from the linked page:

    @detail_route(methods=['post'])
    def set_password(self, request, pk=None):

creating a form for calling set_password. Likely this would require additional arguments passed to the detail_route decorator (such as the serializer used, PasswordSerializer).

See: https://groups.google.com/forum/#!topic/django-rest-framework/e75osh1Wcyg