Add internal option for gear()

[hyperair]

This option allows for the generation of internal spur gears. Internal mode adapts the meanings of several options for use on internal gears:

• backlash is negated for larger teeth, as we're subtracting the gear from a cylinder.
• clearance is added to the addendum instead of dedendum, which pushes out the outer circle instead of pushing in the root circle.
• rim_width is added to outer_radius in order to get the containing cylinder's radius.
• flat renders the gear shape in 2d, ready for extrusion and subtraction.