Brompton rear dropout / to fit regular derailleurs...

###logfile:
2018-06-21 -- v0.8, initial digital beta design
2018-06-22 -- v0.9, v0.8 plus bolt thread engineered
2018-06-27 -- v1.1, v0.9 printed, thread works, all now massive for plastic!

###Intro:
Have you ever wanted to mount a regular bike's rear derailleur to a Brompton?

As an owner of a T5 Brompton, I did. The reason: Older Bromptons lacked the brazed-on mount socket for the gear shifting mechanism, allowing to mount single sprocket derailleurs only. These would be featured by s