Add support for libdns/acmedns #409

Similar to the request made at #395, I'd like to request that the native ACME feature builds libdns with support for acmedns provider. As stated in the current docs, this is not yet the case.

I've read that dns01-providers.txt would allow me to build my own binary with support, but unfortunately in the scope I'd like to use HAProxy, I can't do custom builds (yet). I'm also aware the whole DNS challenge topic is still in the making, but having this would allow for some pretty nifty test setups 🤓