Define supported formats

[zbyte64]

There is a TODO remaining formats in the code:
https://github.com/core-api/python-coreschema/blob/master/coreschema/schemas.py#L22

Projects down the line need a definitive list so that they can use this feature: encode/django-rest-framework#5511

If I were to get a list of the desired supported formats and their strings, I could crank it out. Maybe we should adopt some format values from OpenAPI? https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#data-types