allow convexHull for feature, fix #230, fix #231, fix #226

Fixes a lot of things surrounding convexHull and closed polygons.

* `primitive.convexHull()` now returns `null` or a `Terraformer.Polygon`.
* `Terraformer.Circle` is now closed.
* `Polygon` returned by `primitive.convexHull()` are now closed.
* `primitive.convexHull()` will now handle features.
* Various linter warnings fixed in tests.
* Docs updated with convex hull changes.

@JerrySievert if this looks good tell me and I can bump the version and republish.