Fix problem with sending geojson to geojson.io

Before, if you set --geojsonio to "no" and --kml to "no", nothing would
be done, even if you wanted to send the geojson to geojson.io using
--geojsonio.
Use global 'args.geojsonio' var in 'make_gis()'
Updated README.md with 'Synopsis' and 'Use' info