Compatibility / feature request : replacement for get_default_fields()?

Hello,

I am using [django-hvad](https://github.com/KristianOellegaard/django-hvad/) in my projects and used to have some glue code to make it work with restframework version 2. I used to be able to do this:
```python
class TranslatableModelSerializer(ModelSerializer):
    # other stuff
    def get_default_fields(self):
        fields = super(TranslatableModelMixin, self).get_default_fields()
        fields.update(self._get_translated_fields())
        return fields
````

I am now in the process of switching to restframework v3. Unfortunately, it seems this method disappeared. The new way of getting serializer fields is a huge monolithic method, which is completely impossible to customize, as far as I see it.

I tried overriding `_get_default_field_names`, but it fails at a later point, because additional field names are not known to the new `get_fields` method, causing it to raise an `ImproperlyConfigured('Field name `%s` is not valid for model `%s`.')` exception.

What I would love to see is a that the `get_fields` method from `ModelSerializer` gives a chance for a derived class to provide info (namely, `field_cls` and `kwargs`) for custom fields before raising.

I attached a PR to show what I have in mind.