Add map support

[SamWhited]

Allow user to specify a GeoJSON file, a coordinate, and/or a Mapbox basemap to load in place of a picture. If a GeoJSON file is loaded, zoom to extents. If a point is specified, center on that point (and show a marker unless it's hidden). In thumbnail, zoom to point if specified. Otherwise, show first point from GeoJSON or basemap.

page/site.cover_type: photo|map
page/site.icon: (Maki ID)|hide
page.latitude
page.longitude
page.geojson (URL)
page/site.basemap (Mapbox ID)

Category pages should have a map view as well. Also an "all posts" map view. Might be a pain...