Add min_zoom for important themes

[nvkelso]

The following themes could use explicate min_zoom values that are harmonized across the set of themes and line up with web mapping zooms (but allow float values):

• 10m_cultural/ne_10m_admin_0_countries.shp
• 10m_cultural/ne_10m_admin_0_disputed_areas.shp
• 10m_cultural/ne_10m_admin_0_map_subunits.shp
• 10m_cultural/ne_10m_admin_1_states_provinces_lakes.shp
• 10m_cultural/ne_10m_populated_places.shp
• 10m_cultural/ne_10m_roads.shp
• 10m_cultural/ne_10m_urban_areas.shp
• 10m_physical/ne_10m_antarctic_ice_shelves_polys.shp
• 10m_physical/ne_10m_geography_marine_polys.shp
• 10m_physical/ne_10m_geography_regions_elevation_points.shp
• 10m_physical/ne_10m_geography_regions_polys.shp
• 10m_physical/ne_10m_glaciated_areas.shp
• 10m_physical/ne_10m_lakes.shp
• 10m_physical/ne_10m_lakes_europe.shp
• 10m_physical/ne_10m_lakes_north_america.shp
• 10m_physical/ne_10m_minor_islands.shp
• 10m_physical/ne_10m_playas.shp
• 10m_physical/ne_10m_reefs.shp
• 10m_physical/ne_10m_rivers_europe.shp
• 10m_physical/ne_10m_rivers_lake_centerlines.shp
• 10m_physical/ne_10m_rivers_lake_centerlines_scale_rank.shp
• 10m_physical/ne_10m_rivers_north_america.shp

[nvkelso]

Closing in favor of #202.