Tabletop Dice - 3.0

Minor changes to improve printability on FDM printers. The .scad file uses the experimental roof() module, so you'll need a [nightly build of OpenSCAD](https://openscad.org/downloads.html#snapshots) to use it.

CHANGELOG
- added 45° taper to extrusion (uses experimental roof)
- use alternative underscore character for consistent stroke width
- corrected the orientation of 6 and 9 on d20
- omit dice by setting height to 0 (works better with OpenSCAD customizer)
- improved dice placement