Limit switch bracket/support

**September 2019 UPDATE:**

I moved my Z axis limit switch and needed this piece to be 5mm longer. I updated the OpenSCAD file so that changing the length (and a few other properties) can be done by changing variables. This will also work with the Thingiverse Customizer - if and when the Thingiverse Customizer isn't broken.

This change includes:
* Added variables to the SCAD file so that the overall length of the piece, girth of the platform, height of the shelf risers, the gap between the risers, whether or not the nut trap should be rendered, and on which side of the model the nut trap should be rendered can be manipulated easily
- Rounded the 90 degree angles on the neck for added strength
- Added an M4 nut trap
- Added a rendered model that's 5mm longer and slightly girthier

**Original Summary**

This is an upgrade/replacement for limit switch brackets that attach to 8mm rods, as commonly seen on RepRap and Prusa models.

My first experiences with 3D printing were p