Universal Plug, customizable, enhancement

This is an enhancement of the original customizable Universal Plug.
The new features are:
- supports double-sided plugs
- add rounding plug end
- choose number of slits (including 0) 0
I've changed some additional minor details to make it a bit more beautiful.

As an example, the new version can be used for
- a Lego type connector
- a shelving plug (the small plastic part that's holding the shelves

All pictures and STLs attached here are created from the same customizer.

Link to customizer: https://www.cherrykit.com/thing/p8p1