Improve configurability of leaflet (mapping widget)

Currently, it seems that only the leaflet_tile_layer and leaflet_tile_layer_attribution are configurable.

For private mapping sources is may be necessary to configure other leaflet settings depending on your datasource, for example:

• tile size (defaults to 256, but Google, for example, now uses 1024)
• min zoom
• max zoom
• projection (whether you assume the earth is round, or slightly squidgy - e.g. geoid)

Ideally, anything config exposed by Leaflet, we should be able to configure too.