Automatically look up and return A/AAAA records

[emilazy]

Currently, the configuration file has to either hardcode the IP address of the acme-dns server, which leads to fragility and duplication, or CNAME indirectly though another domain name à la #48.

Since you have to keep up-to-date A/AAAA records for the acme-dns zone with your upstream DNS provider anyway, acme-dns could instead take a nameserver to look up records for its own domain from and import those (whether solely at startup time for simplicity, or refreshed periodically according to the TTL).