exposing proxySocket on socket to support sniffing messages coming from proxy target

Basically I need access to the proxySocket in order to listen to messages coming back.

I realize the implementation is a bit hacky, but it's the only way I could think of ATM. I'm open for other suggestions.

Here is an example use case taken from [chromium-remote-debugging-proxy](https://github.com/thlorenz/chromium-remote-debugging-proxy/blob/792e92a1fe3f8035dd6f78504c2d5880756ce340/index.js#L85-L89)

```js
socket.on('data', function ondata(data) {
    if (socket.proxySocket) {
      socket.proxySocket.on('data', function ondata(data) {
        retHybi.parse(data)
      })
     socket.proxySocket = undefined;
    }
})
```

If there is an existing way to make this work, please let me know as well.
@thlorenz its because of [this](https://github.com/nodejitsu/node-http-proxy/blob/master/lib/http-proxy/index.js#L80) which should be changed. the callback mechanics may need to be tweaked in `web-incoming`