Change Extra.ignore to Extra.allow

[mike-oakley]

Hey there!

I think this library would be a lot more flexible if the models allowed extra fields to be set so arbitrary extensions can be used. This will make it a lot more useful for downstream libraries such as client generators.

I noticed in #9 you mentioned that you preferred to disallow this to show the library doesn't support this, but as far as I can tell, allowing the extensions will not affect the way this library works. Granted, extensions will not have validation or typing, but as long as we add a disclaimer in the readme I don't see why these shouldn't be allowed?

Thanks 😄