Implement automatic extruder fan control based on temperature

This change allows fan outputs to automatically turn on/off when the associated extruder temperature is above/below a threshold temperature. Multiple extruders can be assigned to the same pin in which case the fan will turn on when any selected extruder is above the threshold.

This feature is very useful as the fans will automatically startup regardless of whether you use manual controls or are printing a Gcode file. Today software controlled fans are normally only automatically driven through your slicer configuration, which means you either usually need to remember to turn the fan on manually when you turn on the extruders (if you are using manual controls) or like most people, just wire them to turn on permanently which is noisy.