geojsonio v0.4.0

NEW FEATURES

• Gains new functions geo2topo, topo2geo, topojson_write, and topojson_read for working with TopoJSON data - associated with this, we
  now import geojson package (#24) (#100)

MINOR IMPROVEMENTS

• Updated vignette with details on the GeoJSON specification to the new
  specification at https://tools.ietf.org/html/rfc7946 (#114)