Add a way to override content types

[dbanty]

Add content_type_overrides or similar to the formats accepted by --config so that folks can redirect a specialized content type to one of the implementations that exist in this project.

Specifically, one might solve #592 by adding something like:

content_type_overrides:
    "application/vnd.api+json": "application/json"

See #655 for more info.