Added a new folder for variants with 3 new files

All untested so far! carriagewfan is a carriage that has a 40mm fan
mounting option. jhead_double_mount is a dual j-head mount. Use with (2)
carriages. rod_mounts_20t adjusts the positioning of the y-rods so that
20t pulleys can be used in place of the standard 40t pulleys (requires
624 bearings for the idler).