Just a delta carriage for open belts

I just rewrote it for my build to better accommodate open belts and while I'm at it make it better tunable in scad in case some parameters change. I also added two mounting options — 20x20 pattern and the original one, since I wasn't sure what carriage I'm going to use and added both option to the roller — https://www.thingiverse.com/thing:2749629