Config.txt entry for Steam directory

I've added an entry in config.txt for the Steam directory. Right now it will be used if it's set, or the directory will be auto detected if it's not.

I know there was a similar config entry before that was removed when the Windows registry key started being used, but I don't see any reason to completely remove it. This way the user will always have an option if the registry/hard coded folders fail, on any platform.