s_mute, s_muteWhenMinimized, s_muteWhenUnfocused

minor changes in .gitignore, and default.cfg (see commits)

added s_mute, s_muteWhenMinimized, s_muteWhenUnfocused
all 3 cvars support the following values

0: don't mute
1: mute everything
2: mute all but non-local gamesounds (to strip most of what is optional to the gameplay like menu sounds),
3: mute all but local sounds (great for muting all but chatnotifications and 'the overmind is dying' etc. while being minimized)

only s_muteWhenMinimized is set to 1 by default