Pen plotter attachment for Prusa MK3s

This is a simple mod for your MK3 to use it as a penplotter. The main design objective was for it not to interfere with printing and to not have to assemble or disassemble any parts to change from one to the other.

I tested multiple designs found here and on thingiverse, but most interfered with printing in one way or another, required heavy modification to the E-axis, or didn't hold the pen with enough stability to draw precisely.

I found [this design](https://www.thingiverse.com/thing:3515956) on thingiverse that had the easiest assembly and based the holder on that, then just iterated and tested a bunch of different pen holders in order to have a modular and stable system that accepted multiple types of pens.

The holder installs onto the extruder fan, and just needs a couple of screws that are 5mm longer than the installed ones (I got them from the extras pouch in my update kit). It doesn't block the fan or compromise the assembly in any way, and the E-block is able to home