Improved idler for Folger Technologies E3D Direct Drive

Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:1427181.

I didn't want to have to change the spring tension each time I changed filament, and there wasn't a good way to grab release the idler on the original design. So I loaded the file up in OpenSCAD and added a quick and dirty thumb tab.