SD/microSD Holder remix

I really liked hackermagnet's original holder but my weird little printer always requires tweaks to make things size correctly. I get particularly bad shrinkage on small holes, so I knew I would have to tweak all the screw and nut stuff. In the course of making the adjustments I needed, I ended up cleaning up some inconsistencies in the .scad code, exposing more parameters in a cleaner way in the customizer, and adding a few improvements:

- Corrected customizer syntax and split the parameters into separate sections for Dimensions and Options
- Resized micro SD card slots (they were a bit tight compared to the SD)
- Made SD and uSD card dimensions adjustable in customizer
- Increased default screw hole sizes to allow for shrinkage
- Increased default magnet sizes to take 5mm x 3mm magnets (closest I had on hand)
- Increased spacer size to accommodate larger screw hole
- Added build_plate library to ensure makeability on my small printer
- Made magnets optional (set magnets t