Implemented log callback

[ctrlaltmilk]

This is a first draft I threw together to resolve #94. I'm fully expecting this to be inadequate, so if there's concerns feel free to share them.

[Dacode45]

I like the idea, but I don't want to build another static library. If you can make this work by including your new .h in rgui_wrapper.cpp and rgui_wrapper.h and check this works on windows and Linux, I'll merge