Support blanking the screen with 'b' or '.'

[joshtriplett]

Several popular pieces of presentation software support blanking the screen by hitting 'b' or '.', such as to allow using a whiteboard, or blanking before or after giving a talk. Various presentation remotes provide a blank screen key, which sends either 'b' or '.' depending on the remote. Please consider adding a screen-blanking mechanism, triggered by 'b' or '.'.