BLTouch Chimera mount

Remixed the original file to include a few changes:
- Slots for m3 nuts (use a soldering iron to sink them)
- Hole allowing access to the adjusment screw on top of the BLTouch sensor
- Hole allowing the BLTouch cables to go through the back
- Beefier top plate (less flex)

10 Feb. 2017:
- Added holes for springs,, allowing fine height adjustment

10 Apr. 2017:
- Adjusted the size of the M3 nut's pockets