Filament runout sensor for Tevo Tarantula (and others)

Quick and dirty filament runout sensor. I will be leveraging a RPi3 for this, but there are tutorials on how to wire a runout sensor to the MKS board. This is for 1.75mm filaments only.

For this, you will need:
- Raspberry Pi running Octoprint
- I am running Marlin 1.1.5. YMMV if you are running something else...
- 2x M4x10 socket head cap screws
- 2 M4 T-nuts (like the ones the printer came with) -OR-
- 2 M4 threaded inserts (I used McMaster-Carr P/N 94510A040
- A microswitch. I used a