Laazurus' Migbot Extruder Brace w/chain anchor

Added a chain anchor to laazarus' extruder brace. I found his brace to work the best to keep the extruder relatively true.

Including 2 files - 2b has the anchor closest to the X axis. 2c is away from the X axis. I wasn't sure which mount/where I was going to use on the other end, so I made both.