Double ultimate filament holder for Prusa i3 mk 2

Edit (20th June 2017) - small change to the model:

* top spool holder cutout has been made smooth to remove a stress riser where it would sometimes snap after an impact.
* top spool holder is now mostly hollow to make it more rigid
* vertical arm has a cutout to make it more rigid laterally
* gcode file included for printing in abs on an i3mk2

Filament holder for holding two spools on top of a Prusa i3 mk2. Perfect for multi-colour prints where you need to change the filament mid-run,