Support certificate pinning

It would be a very great security improvement for some node projects of mine, if I could get [certificate pinning](https://www.owasp.org/index.php/Certificate_and_Public_Key_Pinning) running with superagent.

I could relativly easy implement this, If I colud get access to the raw socket via `socket.on('secureConnect')`, but it I have not found a way to do this.

Is there a way to access the socket? If not could you please implement access to the socket?

Start by tracing superagent's Node.js request path to the point where `socket.on('secureConnect')` would be used, and determine whether the underlying socket is currently exposed. The work is done when Node consumers can access the socket needed for certificate pinning, with the behavior verified without affecting browser usage.

Written by the indexing model from the issue text.