restrict TIGER download by county code

[missinglink]

For smaller extracts, it's not desirable to download the whole state's worth of TIGER data.

The config currently looks like this:

{ "state_code": 41 }

... but it would be nice if it also supported:

{ "state_code": 41, "county_code": 5 }

reference: https://www.census.gov/geographies/reference-files/2016/demo/popest/2016-fips.html