Add feature to allow control of hotend fan #8

It would be good to have software / firmware control of the always-on hotend fan. Currently these are just wired to constant power on the board but a controllable pin that cuts power when the hotend is below a safe threshold would be better. That way when a print is complete and the hotend cools completely the fan could shut off automatically, reducing noise and extending the life of the fan.

Not sure if this requires a hardware redesign or not. Would definitely require a corresponding firmware modification. There is precedent for this feature in Repetier firmware w/ RAMBO boards.