Flexible shaft coupling holder

`This piece holds a flexible shaft coupling, such as those frequently used in 3D printers. It allows to drill into it without the coupling wobbling.

I needed this for the [Bubblegum CNC kit](http://www.thingiverse.com/thing:33799), as the conversion requires 5mm to 4.5mm coupler, which proved impossible to find. The solution then is to get a 5mm to 4mm one, and enlarge the hole.

The piece needs to be made to exactly fit the coupler's external size. I found both 18mm and 19mm couplers, so I included files for both. It's easy to produce whatever size is required from the OpenSCAD source.