Add support for disabling log from settings

Enabled by default for now because of development. The ability to disable the log will definitely be important to non-developers if Citra ever becomes an emulator capable of running commercial games. The average user doesn't want to see the ugly mess of output that Citra creates. If Citra ever does become a fully fledged 3DS emulator with a UI and everything, the option to enable the log can be hidden away into development settings. This is obviously not a very important feature at the moment, but to be honest, one of the reasons I'm doing something simple like this is to get myself back into C++ :P
I'd say there should be some way to log critical errors in configuration handling. Those can just be printed in some "default" manner (i.e. stdout) until the logger has been initialized "properly". (You could argue that this is useless on Windows, but I don't think there's a good way of notifying the user about errors there unless we want to introduce some ErrorDialog function...)