Make attribute getter function configurable

[sloria]

Currently, marshmallow.utils.get_value is used pull values from many different types of objects (both simple and complex types).

It may be useful to override this behavior, e.g. via a class Meta option, when you know exactly what type of objects you will be serializing and how to pull values from them.

I see two use cases for this:

• Handling objects that get_value will not work with
• Optimizing serialization