Public key pinning support

I see that cert pinning is supported, but how much extra work would it be to add a method for pinning just a public key?

This would be much more convenient in most cases I think, given that key pairs are generally long-lived while certificates change on a regular basis — more so now with the rising popularity of Let's Encrypt which issues 90-day certs. With public key pining, I can just commit the public key (or hash) as a static file once and forget about it, whereas it seems like cert pinning would require me to implement additional logic to ensure that the latest version of the cert is always stored locally.

(Note: I've looked at the readme, but haven't yet used this library or done much digging into how Android, iOS, or the linked libraries handle TLS pinning; so my assumptions about what is and/or can be supported may be completely off.)

README にある既存の certificate-pinning の説明から始め、次に issue で言及されている Android および iOS の TLS-pinning 統合を調査します。リンク先のライブラリが公開鍵 pinning またはハッシュ pinning をサポートしているかを判断し、必要な API と検証動作を定義します。実装可能なスコープが決定されれば完了です。