Support making visor variant C.

Update Makefile to build visor-C.stl.
Add lenses.scad to dependencies for everyone.
The current code to enable variant C is nasty. Need to refactor.
Add STL for the new visor variant.
Clean up elastic mount logic.

Remove the need to pass the variant down this far.
Still not perfect, but it's cleaner than the first pass.
Update README for variant C.
Clean up visor body description.

File no longer depends directly on variant.
Factor out parameters that may need to be calculated later.
Add documentation to new modules as needed.
Correct height of straps on new body.