Enable the user to specify a callable as a global log property

[piotrmaslanka]

If a callable is given, it will be evaluated at _log time. This way you can for example add current trace IDs (if you're using tracing).
If the callable returns None, it won't be included at that time.

[tintoy]

Thanks for contributing- this looks great! 🙂

I’ll get it merged sometime this morning.

[tintoy]

Thanks again 🙂

[tintoy]

Published v0.3.18.