Printrbot Metal Plus Lever Extension

This is a lever extension for the Printrbot Metal Plus, single-extruder model 1412. This is for the Version 1 extruder, not Version 2. It's roughly based on the design for the [Printrbot Simple lever](http://www.thingiverse.com/thing:566321) by SlovenskaPosta. The Metal Plus has limited space between the print head lever and the side z-axis rods. It also has a hex bolt-head between the extruder motor and the existing lever, so I had to recreate the 3D model with the sizes adjusted. I also includ