Improved Low-profile Endstop Holder

This design is based on [this](https://www.thingiverse.com/thing:919432) endstop holder. I have made several improvements:

entire endstop holder is longer to prevent play
rounded profile to fit onto smooth rod more snugly
optional captive nut traps


These changes were made after noticing that my y-axis kept crashing into the end due to the original design being far too loose.

M3x16 bolts with M3 nylock nuts recommended, normal M3 nuts and longer bolts should also work.

Please note that due to the tighter fit, putting the endstop holder on will require quite a bit of force the first time you install them, the plastic gets more pliable the more you take it on and off.