add zip code support

[DanielJWood]

One way to do it that would be as follows:

• get zip shapes
• get centroids
• map each centroid to nearest lat/lon of place we have in our current gazetteer
• join to those places.

It might be more performant to include the zip list separately and join in the browser?

Some possible issues:

• for large cities, like DC, there are lots of zip codes per entry in the gazetteer.
• for large zip areas, a centroid may be far whatever population center is core to the zip.