Pen holder for Anet A8

Pen holder for the Anet A8 and other Prusa i3 derivatives. Screws in place of the extruder motor fan. Max pen diameter: 12mm Screw the pen in place with M3 screws and nuts. CHANGELOG: v3: Two screws instead of one to hold the pen for better stability. v2.1: Better spacing of screw holes. v2: Puts the pen closer to the nozzle.