Add alternative port support for upstream server

[finwo]

Just like michael105 did in CupIvan#6, I want support for a custom upstream port.

Unlike michael did, I want to support udp port notation instead of adding an extra configuration field.

So, instead of doing this:

{
  dns:      "8.8.8.8",
  dns_port: 53
}

I want to do this:

{
  dns: "8.8.8.8#53"
}

[finwo]

Implemented by #10