Compact Bowden Extruder Derivative

I found a nice simple cold-end design (thing #275593) and made some changes based on my needs.

This version uses a different bearing and drive pulley - the same parts that come with QU-BD's one-up / two-up printers (this is for an upgrade to one of those printers, re-using as many parts as possible).

Also added a thumb lever to help with changing filament, switched the quick-connect coupling for a M4 nut coupling, added a taper to the inside of the output channel to help with threading filament through, and various other minor alterations.

Updated: v2 of design posted