VR gun stock for HTC Vive (v2.3.6)

**Updated to v2.3.6 - improved ergonomics** also repaired the problem mesh on the rail stl file.

This is a complete redesign of [bigwoody003](https://www.thingiverse.com/thing:2416753)'s fine stock which I needed to hack down to fit my 12 x 12 cm printer bed. I simplified it for easier printing and remade the cups to precisely fit the HTC Vive wands. I also beefed up the sliders. I didn't make any Rift cups, but the ones from bigwoody's design should fit as the stocks share the same top rail