Content Negotiation Improvement Proposal

This is a rough draft of a proposal for improving Django's support for content negotiation based on previous discussions on django-developers and work by @tomchristie. Currently it's fairly light on technical details but this process is very new and I'm not sure how much as needed at this point. Mostly I just want to revive this discussion and iterate on the details if it is agreed that this is something that would be an improvement to Django.