parameterizable belt tensioner

This is a belt tensioner I developed to assist in tracking down vibration problems on my printer. It has [may have] a knurled knob for reproducible adjustment.

Update 03 April 2018: Modified to work with 20t toothed idler, M3 or M5 spindle. For this size idler the knurled knob around the m5 nut is too small to be useful. Instead I just use a flathead screwdriver from the sides to turn the nut. I advise using a silver nut so you can mark one face with a permanent marker and count the turns so you have some choice over belt tension. This version is shown in the photo with the blue part, with the belt feeding under the gantry and using my [GT2 tooth side belt clip](https://www.thingiverse.com/thing:2848858).

The parts should be easily available but as supplied it is designed specifically for an OpenBuilds/Ooznest 'smooth idler pulley' (22mm diameter, 16.2mm high) and to fit on 2020 v-slot extrusion configured for pulling an OB gantry plate on wheels. [20t idler parts added wit