Add option to specify DNS server

[phdoerfler]

Thanks for this tool!
One key feature it's missing IMHO is allowing to specify a DNS server.
Even better: As a default it could use either the wireguard interface address assuming/if there's a DNS server running on there, or take what the mikrotik is using for its DNS queries. In my case my mikrotik uses a different DNS server but it also runs its own, caching the other one and I use this own caching DNS server for my wireguard peers: The .1 address.

Thoughts?

[dsm150]

if I don't manually add a DNS=8.8.8.8 entry in the generated config, then there is no internet on the client. If I understood the documentation correctly, then WG does not send all traffic to the tunnel without a DNS= entry in the configuration.