Enable GeoJSON integration via <map-link rel=features type=application/geo+json tref=...>

[prushforth]

It should be possible to add a "permanent" layer of GeoJSON to the viewer. Instead of doing that at the level, we could implement it via the <map-link rel=features> method. What's more, if the json is a "feed" and includes a "next" link, the client could pull the feed accordingly e.g. this feed contains a rel: "next" in its links.

It should be fast, so it probably shouldn't go through the geojson2mapml api, but instead just render the layer. We will need to figure out how to style the rendered content with "CSS". It probably should use the canvas API, and forego conversion to SVG, even, tbd.