Rep2 Filament Extruder Guide

**Update: Rev2 files have more clearance at the top of the motor so the extruder arm upgrade doesn't hit it as easily. Also, there are some weight-saving cutouts that don't significantly do anything but look kinda cool.**

A replacement Filament Extruder Guide for the Rep2. It's lighter and has better airflow.

The main thing is that it allows air to exit out the left side of the heat sink. The material that was previously there was not adding much to the strength of the piece, so I remo