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.)

从 README 中现有的 certificate-pinning 描述开始，然后检查 issue 中提到的 Android 和 iOS TLS-pinning 集成。确定所链接的库是否支持公钥 pinning 或哈希 pinning，并定义所需的 API 和验证行为；确定出可实施的范围即表示完成。