Support HPKP

[daurnimator]

RFC 7469 Public Key Pinning Extension for HTTP

Useful links:

• https://developers.google.com/web/updates/2015/09/HPKP-reporting-with-chrome-46
• https://community.letsencrypt.org/t/hpkp-best-practices-if-you-choose-to-implement/4625

[daurnimator]

Chrome is removing support. https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/he9tr7p3rZ8/eNMwKPmUBAAJ?hn

[RyanSquared]

Is there any intent to move to or adopt what is listed as the replacement, Expect-CT?

-- Edit: I should really read (#83).

[daurnimator]

While firefox still supports HPKP (and chrome hasn't actually removed it yet) I'd still accept HPKP support into lua-http.

However it has entirely dropped down my own priority list.