req: Let's Encrypt DNS-01 challenge support

[eelkevdbos]

This challenge allows applications behind a firewall that restrict access based on IP-address ranges to still have the means to validate without knowing Let's Encrypt's IP ranges (they do not provide any range of their infra by design).

Since this project is a 'minimal' proxy, I could see that this kind of feature would not be in scope, but I am wondering if there are any plans to extend to features like this - or have a pluggable infrastructure that would allow the community to build something like this.

[erka]

I believe this is a part of #30