Style maps using simple style spec

[eyeseast]

Github does it so we can, too.

Here's the spec: https://github.com/mapbox/simplestyle-spec/tree/master/1.1.0

Not sure if this requires using the old mapbox.js. Would rather not add a dependency, especially an old one.

[eyeseast]

Here's an old mapbox example, using Leaflet's GeoJSON layer: https://docs.mapbox.com/mapbox.js/example/v1.0.0/geojson-simplestyle/

[eyeseast]

Would be worth allowing a default style defined in metadata.