`solid.utils.extrude_along_path()` should accept planar `shape_pts` in any orientation, not just XY

See Issue #148.  If `shape_pts` defines a polygon, `solid.utils.extrude_along_path()` should be able to use the normal of that polygon (or the normal of the first 2 points and the centroid of the shape, say, if the polygon isn't planar) to determine which direction to extrude, meaning we wouldn't limited to points in the XY plane, as is currently the case