Support overriding the printer DNS IP address

[lukefor]

Fantastic project, just what I needed to expose my printer from one network to another via NAT. Windows network printing just worked but Airprint needed this extra helping hand

Just one small feature request: I had to hack addr in resolve.js onLookup to a different IP address, as I am NATing ports to the printer. It would be really great to expose the A record IP address as a command line flag in case it differs from the IP that the local machine running airprint-proxy uses for the printer