Fellow Prismo cup adapter

##Universal cup adapter for Fellow Prismo

I love the Prismo and have it on my Aeropress 99% of the time. The only problem is it doesn't fit most of my cups and it quite unstable while pressing down which can (and has) lead to **catastrophic** failure.

Hence this adapter.

As an added bonus, it also provides a better grip for twisting off a stuck Prismo.