Master mono

Added an option for two channel mono output. 

Mixxx has already a mono down-mix when using only one sound card channel.  
But it you need to feed a stereo amp with the mono signal you need a special cable. 

With this patch, you can switch beween Mono and Stereo without tweaking the hardware. 

This feature is required for outdoor gigs, where a Stereo effect is not possible due to the speaker positions.
I would really prefer to call the CO "mono_mixdown", even if we don't reveal this word to the user.