allow for squared corner plates?

[wkearney99]

I've got some items that would work better on squared-corner plates, instead of rounded.

In poking through the code I can just brute-force change line 599 from cylinder to cube, but that doesn't address the inset portion. Which is perhaps just as easily brute-force removed with comments from line 604 to 616! Doing the same for the inset doesn't work, as it doesn't properly translate the cube such that the inset dimensions are correct. I'm no openscad expert, so I'm out of my depth on figuring out what your code is doing.

It'd be fantastic if there were parameters for rounded vs square corners, and inset or not.

Also being able to change the base shelf extension depth would be handy.