Feature request: user defined message handler

[RalfKubis]

Rather than always routing messages to stdout or stderr [fprintf(stderr,...)], it would be great to provide a mechanism that enables the client to register a notification callback function similar to

http://doc.qt.io/qt-5/qtglobal.html#qInstallMessageHandler

This would enable the application to integrate the library into the applications logging facility.
The default message handler could still dump to stderr.

[jmcnamara]

That's a good suggestion. I'll look into it.