Improvement: allow to use custom LookupIP function

[klingac]

Sometimes we need to use other LookupIP() func then default net.LookupIP() func. This is typical, when we need to use custom DNS resolver - e.g. hashicorps consul, etc.
It should be very helpfull to be able to setup cutom function here

func (r *Resolver) Lookup(address string) ([]net.IP, error) {
  ips, err := net.LookupIP(address)