Set retry-after header in default on_deny

[mathiasose]

This is very much a draft PR, I couldn't get the tests to run locally so I have no idea if it works as-is or needs some more work. Need some help from a maintainer to get this ready.

---

From MDN:

The Retry-After response HTTP header indicates how long the user agent should wait before making a follow-up request. (...) When sent with a 429 (Too Many Requests) response, this indicates how long to wait before making a new request.

This seems pretty easy to do in the on_deny handler if we pass scale as an option. Since retry-after is usually given in seconds we do integer division on the ms value, then cast it to a string and set it as the header. Done! In the same vein we can also make limit another option that custom handlers can use if they want.

[JuneKelly]

Awesome! Yeah I think this feature makes sense. I'd be happy to merge with a few basic tests. I can take over from there and write more tests and documentation.

Thanks! :tada: :two_hearts: