Support line and area interpolation

[jonmmease]

Currently line marks use straight lines between points. Vega supports a variety of line interpolation options.

One of basis, bundle, cardinal, catmull-rom, linear, monotone, natural, step, step-after, step-before.

These are based on d3-shape, and implemented in https://github.com/d3/d3-shape/tree/a82254af78f08799c71d7ab25df557c4872a3c51/src/curve. I think we should be able to implement these based on lyon.