Manual feed extruder knob (NEMA17)

Lightweight (and low inertia) knob for easy manual filament loading/unloading.

Designed to tight fit onto NEMA17 shaft, but may be secured with set-screw M3.

Works best with [Compact bowden extruder](http://www.thingiverse.com/thing:275593) and related remixes.

UPD: Version for stepper bore 8mm added (for NEMA17 with planetary gearbox)