Implement automatic cold-end/extruder motor fan control based on nozzle temperature

This change allows fan outputs to automatically turn on/off when the associated extruder hot-end 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. This means if you are using manual controls to preheat your extruder you either need to always remember to also turn on your cold-end fan (manually) or like most people, just wire them to turn on permanently which is noisy.

Note: this feature is intended to control your cold-end/extruder motor fan not a variable speed extruded filament cooling fan.

This pull request replaces https://github.com/ErikZalm/Marlin/pull/390 - this pull request is decoupled from the larger M600 related pull request 378.