[NOT READY] Use Mapnik 3.x expressions

Uses Mapnik 3.x to dynamically render via a GeoJSON `FeatureCollection` + map template.

Todo:

 - [x] This uses SVGs and dynamic transforms, just as a proof of concept. We should instead use pre-generated images for full fidelity and because they will render faster (mapnik caches bitmaps and not SVGs).
 - [x] Retina support will need to be added/tested.
 - [x] All GeoJSONs will need to be passed to mapnik as `FeatureCollections` (as mentioned at #5)
 - [x] If this approach looks good to @tmcw then we can refactor out of the tests and into a lib
 - [x] Resolve why tests are passing but coverage is failing

/cc @yhahn @artemp