Add GeoJSON features with simplestyle styles in a tilelive pipeline.

npm install --save tilelive-overlay

• overlaydata:https://GEOJSONSTRING
• overlaydata:https://2x:GEOJSONSTRING (retina)

Provides a overlaydata:https://GEOJSONSTRING scheme for tilelive, which transforms GeoJSON into a Mapnik stylesheet using geojson-mapnikify, and then provides raster tiles of it.