Ultimaker 2 Replacement Knob

A new knob for the Ultimaker 2 which provides a better grip.
Based on [Ultimaker 2 Knob](https://www.youmagine.com/designs/ultimaker-2-knob--2).

I redesinged fillets a bit and made it a little smaller in diameter because original design turned out to turn a bit strong, at least in my Ultimaker 2.