mapboxapi 0.4

• Enhancements to mb_matrix() which now allow for distance-based matrices instead of the typical travel-time based matrices;
• Enhancements to static basemap functionality, including a new function tm_static_mapbox() to use Mapbox static maps as tmap basemaps;
• Various bug fixes, documentation improvements, and internal performance improvements;
• Added a NEWS.md file to track changes to the package.