Proxy class missing capture socket

[justinrcs]

According to the Zeromq API, a capture socket is supposed to be an optional 3rd argument for the ZeroMQ Proxy class (which it was for 5.x): http://api.zeromq.org/4-2:zmq-proxy

This eliminates the ability to monitor or log messages flowing through a proxy.

[rolftimmermans]

Makes sense. A PR would be welcome!