Better Chicago and Cook County borders

Rename the ``chicago_geojson_from_shp`` management command
``border_geojson_from_shp`` and update it to take a second
argument for a Cook County shapefile.

Update the command to use the cartographic shapefiles as they
remove the offshore areas of Chicago and Cook County.

Update the README to reflect the new command name, the additional
argument and the new datasets.

Addresses https://github.com/sc3/cook-convictions/issues/133