Make MapService more re-usable for displaying maps with different starting centers/layers, etc

[benglass]

Consider subclassing map_render.js for specific maps (the only one we'd need to start would be Vermont). Currently vermont lat/lon center and zoom level is baked into the map renderer.

• Allow passing in of center as an option
• Allow passing in of zoom level as an option
• Abstract the logic for determining which tile layers to use (OSM, Leaflet, etc.) and make it a configurable option