Labeled Filament Test Cube

This is a 3D model based on [sch00f's filament test cube](https://www.thingiverse.com/thing:2166102). It's intended both for calibration and to illustrate how a filament works in a few different situations.

Features of the cube include:

* Each side is 20 mm
* The sides are labeled according to their axes
* The circular hole in the back is 10 mm in diameter; it serves as an overhang test
* The rectangular hole in the back is 5 mm across and 4 mm high; it serves as a bridging test
* The vertical cylinder is 6 mm in diameter, with a 2 mm diameter hole in the middle
* The vertical hole is 6 mm in diameter, with a 2 mm diameter post in the middle
* Text giving the filament type and brand is embedded into the bottom of the cube (this is the principal addition over sch00f's model)
* The entire model is parameterized in OpenSCAD, mostly to facilitate the filament info text, but also allowing for easy scaling.

I've enabled the customizer on this thing, but it really doesn't do a grea