Add a Fan or Jumpers for a Fan

[krkeegan]

I know we are limited on amperage but a small fan would be great. The fan could tie into the temp probes I proposed in #34 to speed up and down as needed. Or in a simple scenario just turn on when the firmware is active.

[blurfl]

"just turn on when the firmware is active." That could be as simple as a 'startup block' like grbl implements to execute at startup. gcode M106 sets fan speed, M105 reads 3D printer bed temperature - these could be used with the daughterboard I suggested in #34