Foldarap Nema14 holder with T-Slot 2020

It holds a Nema 14 motor with a T-slot 20x20. It is based on the [original holder](https://github.com/EmmanuelG/Foldarap/blob/master/hardware/stl/y-motor.stl) for the [Foldarap 3D printer](http://reprap.org/wiki/FoldaRap) by Emmanuel Gilloz, but is remodeled to fit the entire side of the motor and avoid the [bending of the piece by tension of belt](https://flic.kr/p/y1Ldvs). GPLv3 licensed like the original one.