[feature request] customize foreign key properties (*_id)

[orenelbaum]

There's already this issue #25 for customizing the id field.
I want to be able to customize the *_id fields as well.

Is your feature request related to a problem? Please describe.
The problem is I can't get the schema I want, I want to have a Post type with author_id that points to a User type, but this is only one example, it's a common use case.

Describe the solution you'd like
Being able to customize the *_id field and somehow provide metadata about which type the field points to.