Client application should support pipeing

[benjamg]

Currently it isn't easy to use the zmqpp client application to pipe data in a bash shell.

Ideally this needs to be reworked to allow data to be piped both in and out of an instance and so allow simple shell scripts to create zmq devices.

Some thought will probably need to go into this to work with multiple part messages, currently my thinking is stick with newline as a part double newline as a message (similar to http) when the multipart option is enabled, and just newline for a message otherwise.