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
Anyway, you can merge this now and I'll make a separate PR afterwards to join all of these, as well as fix some of the other little quirks with the config system.