Add HTTP Public Key Pinning support

[dmp1ce]

Suggested in #23.

[dmp1ce]

I'm not such how to setup HPKP to be honest. I have read a little about it here: https://raymii.org/s/articles/HTTP_Public_Key_Pinning_Extension_HPKP.html

I think we might need to ask Let's Encrypt for the public.key in order to generate a fingerprint for the certificate and the public key. That part can be done in this project.

Generating the nginx configuration for HPKP shoud probably be done in jwilder/nginx-proxy if the public.key is availble. Pull requests are welcome.