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 / cl