Flexi-felixstruder for Felix 3.0

This is a modification to the extruder arm of the Felix 3.0 so that it can extrude flexible filament painlessly. It is based on the Flexystruder design by Lulzbot and this adaptation by Fabsterdam: http://fabsterdam.com/3dprinting/building-a-flexystruder/

The idea is to simply let the PTFE tube go directly up to the extruder entrance with just a half circle groove for the driving gear.