Top mounted filament guide for Wanhao i3/Maker Select

### Why?
When the filament is mounted on the control box (which is safer and more stable than mounting it on top of the Z bridge) the filament grinds against the frame and can cause under-extrusion.

This filament guide is optimized for use with the spool on the control box.

### Features

* Mounts using existing holes on the Wanhao i3 V2.1 and probably all recent Maker Select models

* The filament hole is beveled outwards to provide a smooth path from all directions

### Open source