FilaRailSpoolHolder

I use the original [FilaRollSlider design](https://www.thingiverse.com/thing:2486632), but if I don't plan to use a spool for some time I'll vacuum seal it to protect it from moisture. Depending on the type of spool the filament manufacturer uses, the process can cause the sides of the spool to bend a bit, which means the spools don't roll as smoothly.

I combined the rail portion of the original design with [Zweben's Spool Holder](https://www.thingiverse.com/thing:2026895) to come up with something that slides onto the rail and holds a spool by the center hole for those spools that need it.