Fixed up the mute command in /etc/freeconf.conf

Before it didn't work with multiple active sinks, now it loops through
each active interface and toggles it. This could be changed to detect
if any interface is unmuted and then mute them all, or unmute all of
them, which is probably more correct
Added note about cinnamon

I had to change my /etc/freekey.conf to use
'cinnamon-screensaver-command', and left that in there in the last
commit. I changed it back to 'gnome-screensaver-command', but left a
note about changing if you're running Cinnamon.
Merge pull request #2 from atomictom/master

Fixed the /etc/freekey.conf file when using multiple interfaces