Parametric Circa/Disk-bound Cover or Bookmark

I actually made this so that a calendar would have a hangar, not as a book cover, but it generalizes pretty well to book cover and bookmarks, especially with the custom top offset.

Included files:

- the `.scad` file
- a "half letter" book cover, designed for use with 8.5"x5.5" paper
- the calendar hanger, designed for 8.5" wide printed-off calendars
- a "top tab" bookmark, in the style of Scotty-G's [Disc System Page Marker](https://www.thingiverse.com/thing:2331155)