Support rate limit headers as defined in the IETF draft

[davidor]

This is the draft: https://datatracker.ietf.org/doc/draft-ietf-httpapi-ratelimit-headers/ @unleashed is a co-author, so he can help answer any questions about it.

[unleashed]

In the future there is a very good chance that the header values will require the use of Structured Field Values - so while a simple implementation using the draft as-is now can do for a while, later on we'll want to use a library (or create one) capable of outputting SFV.

[alexsnaps]

See #158