Add ability to make an online display active or deactivate a display

[jhford]

It'd be neat to be able to turn a display that is plugged in but off on, or an active display off.

screenresolution 800x600x32 off
-should turn display 0 to 800x600x32
-should deactivate display 1
screenresolution off 800x600x32
-should deactivate display 0
-should set display 1 to 800x600x32
screenresolution skip off
-should skip display 0
-should deactivate display 1