Get :points prop to work with Map-component

[luucvanderzee]

The :points prop, which exists on all Path-like marks and the vgg-multi-point mark, can currently not be used with vgg-map. The only way to use :points is to enter the centroids manually and use the parent Section's :scale-x and scale-y props. For the sake of completeness, it would be nice to also support using this with vgg-map and inline scaling. The biggest hurdle is that we would need to define two scales- one for the x-dimension and one for the y-dimension. For that reason it is maybe a good idea to combine this with #102, which also involves using multiple scales within a single prop. Thoughts?