Handle data callback

[martinlevesque]

Hey,

Great package! It would be nice to add a config handler whenever any data on a hostname is transfered, example:

let myConfig = [
    {
        name: 'app-1',
        hostnames: [ 'test.myhost.com' ],
        endpoint: { host: '127.0.0.1', port: 3001 },
        dataHandler: function(socket, data) {
             // do something...
        }
    },
      {
          name: 'app-2',
          hostnames: [ 'test2.myhost.com' ],
          endpoint: { host: '127.0.0.1', port: 3002 }
      }
  ];

I need it to calculate the bandwidth usage per hostname, so I would sum the data bytes transferred. It could be useful for other purposes.