VM Extruder

After using a gregfrost extruder on my Prusa for a while, I decided I wanted to make a few tweaks to the basic design:

- replace the hobbed bolt (I found it too strip-prone) with a large-diameter filament driver that requires no machining

- a spring-loaded over-center latch on the idler arm, allowing opening the idler without unscrewing anything

- an idler arm that opens flat to provide even better access to inspect & clean the filament drive

After assembling it and running a few tes