Generate API files bidirectionally

[waldoj]

Right now, we're creating a file for each GNIS ID, but not for each FIPS code, which is to say that this is useful for FIPS -> GNIS, but not GNIS -> FIPS. I'm not sure whether any GNIS IDs are identical to any FIPS codes. If not, then they can live side by side. But if they are, then we need to namespace the files, e.g. /gnis/12345.json.