make logging functions dynamic

[Vortex2Oblivion]

makes functions dynamic so you can override them with your own behavior

[Vortex2Oblivion]

openfl/openfl#2846 will also need to be merged

[player-03]

Other approaches to consider:

• If you just need to override this class for your own project, you could shadow it. That is, add a file named lime/utils/Log.hx to your classpath, and put whatever you need in there. It will then be used instead of Lime's version, even by Lime classes.
• If you want to suppress log messages, you can do that with level.
• If you want to print to somewhere else, we could make a variable like public static var println:String -> Void that you could set, and all functions would call.