Customizable belt loop keeper

I always lose these belt loop keepers which retain your pants' belt, so made a customizable one in OpenSCAD. It requires [BOSL2](https://github.com/BelfrySCAD/BOSL2) for those nice bevels.

Changeable parameters:

`BELT_WIDTH` the width of your belt

`BELT_THICK` the thickness of your belt

`LOOP_WIDTH` how wide the keeper should be

`LOOP_THICK` how thick the keeper should be

`ADJUST` to adjust for tightness (the default 1 mm should be just fine).