Add API to redirect print() and friends

[swisspol]

On Android and WinRT at least, you do have stdout and stderr of course but they lead nowhere. You have to use platform specific logging APIs to actually see the output.

It would be great if v7 had some sort of "hook API" where one can install a callback to print to stdout or stderr to override the default implementation.

That or that the code is aware of being built on Android JNI / WinRT and use the appropriate logging APIs :smile: