Aluminum Extrusion Endcap Customizer

This is based of of the "end cap for 3030 extrusion" thing by jag. I couldn't get his to work correctly, I think it was not adding clearance for the thickness of the aluminum for the slot which made it not fit, now matter how much "undersize" I used in is OpenSCAD file.

What changed:

* I did some reorganizing of the .scad file to make it easier to follow for me
* I changed undersize ... instead of the overall undersize of a particular length/width, undersize is now the "margin / gap / clearance" around the inserts of the model. For example 0.2mm is the amount the width is reduced on each side of the insert, not the reduction in total. For 3030 use 0.2mm and for 2020 use 0.1mm for a good fit.
* I added explanations and created a customizer so you could edit yourself on Thingiverse

The default values for the customizer are based on Misumi HFS6-3030 extrusion (found at https://us.misumi-ec.com/vona2/detail/110302686450/). I uploaded the Misumi dimensions diagram and highlighte