Have Geomapper accept "county" for "fips" conversions

To convert something to counties, the geomapper requires the argument be  `'fips'` and not `'county'`, which makes it hard to do something like

```
for geo in georesolutions:
    geomapper.add_geocode(df, fromcode, geo,....)
```
since we need `'county'` to exist in the output filenames, you end up having to do a condition statement or use a dict to map `'county'` to `'fips'`