Customizable Shelf Pen Holder

This is a customizeable pen holder that lets you define the number, size and spacing of the holes in a hanging pen holder that clips onto 2cm high shelves from the [Container Store](https://www.containerstore.com/s/driftwood-melamine-shelves/d?productId=10018499)
The original design in OpenJSCAD is [here](https://github.com/mscherotter/shelfpenholder) and a [video](https://www.youtube.com/watch?v=NxCsi96UtqE) showing how I made it.

7/19/2020 - Added shelfThickness parameter and filled corners near clips to improve strength.