Add a -D --define option to set libf3d options directly

[mwestphal]

Is your feature request related to a problem? Please describe.
F3D CLI options are great but being able to set libf3d options directly would provide more flexibility to users

Describe the solution you'd like
Add a -D or --define option that would support the following:

./bin/f3d -D=render.effect.ambient_occlusion=true -D=model.scivis.array_name=name

(specific syntax tbd depending on what cxxopts can do)

Additional context
#1569

[mwestphal]

related to #1530