Release v0.2.1

Changed

• osm is now the default basemap, replacing esri-light. OpenStreetMap labels cities and states at the zooms a cross-country route sits at, where the Esri gray canvas drops them, so a map reads without extra annotation. esri-light remains available for a quieter frame.
• OpenStreetMap is credited as "OpenStreetMap contributors" in the source line, the attribution its license asks for.

Fixed

• Mapbox backgrounds returned a 404 and rendered a bare route with no basemap. Mapbox style IDs carry a version suffix, so --background mapbox-light was requesting the style mapbox/light, which doesn't exist. Short names now resolve to the current version (light to light-v11, streets to streets-v12 and so on); versioned IDs and custom owner/styleid styles still pass through untouched.
• A basemap that fails at tile-request time now falls back to the default rather than saving a map with nothing behind the route, and the source line is rewritten to credit the basemap actually drawn.
• Corrected __version__, which reported 1.0.0 in the 0.2.0 release.