consider adding line.points

[ianstormtaylor]

This way APIs could accept an interface of { points: Point[] } and be made to work with both Polygon-type shapes and Line-type shapes easily.

This would also allow for destructuring line.points to quickly renaming variables, for example:

// could be...
let [a, b] = line.points

// instead of...
let a = line.p1
let b = line.p2

(I'm just opening issues as I use the library and run into problems or inconveniences, but let me know if you'd prefer I not open these kind of ideas issues.)