HTTP Public Key Pinning (HPKP) support

[rugk]

I wonder if you support HPKP. In case not, it would be awesome if you'll add support.
E.g. Posteo supports it and it can be easily implemented as it is just a HTTP header. However the result is a huge security improvement making MITM attacks much more difficult.

Here more information about HPKP:

• https://en.wikipedia.org/wiki/HTTP_Public_Key_Pinning
• https://scotthelme.co.uk/guidance-on-setting-up-hpkp/
• https://developer.mozilla.org/en-US/docs/Web/Security/Public_Key_Pinning