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
All logging calls silently do nothing if the logger isn't initialized, so this shouldn't cause any *serious* problems I don't think.