Add defaultdict support

[AndrewSergienko]

Issue #216

[zhPavel]

The code is quite good now. But there is need a documentation at specific-types-behavior.rst file. Desciption should contain link to function that can be used to override default_factory

[zhPavel]

It is also necessary to add a fragment of the changelog with a description of the new feature. You can find instruction to creating such fragments in docs/changelog/fragments/README.rst