Z-knob stop for MakerGear M2 dual

This is a second attempt to create a useful locking mechanism for the MakerGear M2's Z-axis knob. The first version, http://www.thingiverse.com/thing:873378, does not allow the M2 dual extruder carriage to move all the way to the left. This version stays clear of the extruder carriage in all positions while keeping the feature of releasing itself if the Z stage is homed while the lock is still in place. It has the additional feature that the lock handle can be placed upside-down on the cap and makes a convenient handle for hand-cranking the Z stage.

Updated July 22; knob now has a small overhang at the top, which helps stop the lever from twisting in the locked position, and also helps keep the level from slipping out when using it as a hand crank.