Add County spatial data

Add a County model to hold County data from the TIGER shapefile.

Added an ``in_cook_county`` flag to the ``CensusPlace`` model.

Add a ``flag_cook_county_places`` management command to set the
``is_cook_county`` flag on CensusPlaces that intersect Cook County.

Renamed ``export_chicago_geojson`` management command to
``export_border_geojson``.   Updated the command to load the Chicago
boundary from the ``CensusPlace`` model instead of trying to aggregate
it from the border of the community areas.  Updated the command to add
both Cook County and Chicago's boundaries in the GeoJSON.

Update README to reflect all of this.

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