UPnP and External IP

[hayesgm]

Currently, we advertise our node as enode://{public_key}@127.0.0.1:30303 unless you pass in an EXT_IP_ADDRESS environment variable. If you're behind a gateway / router, even if you put your external IP address, it will still be impossible, based on NAT traversal, to connect inbound to your node (though, you should be able to connect outbound okay). UPnP is a protocol that allows a machine on a private network (behind a router) to accept inbound connections to a given port. We should allow our node to use UPnP to allow inbound connections, and we should properly set our external IP address in our node address without specifying it on the command line.

You can see the following options in Geth/Parity, which we should mimic:

--nat=[METHOD]
        Specify method to use for determining public address. Must be one of: any, none, upnp, extip:<IP>. (default:
        any)

We actually already ported a upnp erlang project to Elixir here: https://github.com/exthereum/nat_upnp (nat_upnp on hex). We could get that library up to date (the erlang library now has an option to get your external IP address that our fork doesn't), and use that for UPnP and external IP address discovery.