refactor and add CSVs for additional zips

[philvarner]

1. Primary fix is for Some zipcodes with PHZ data are not in zipcodes.csv file, and therefore have no api json file #35 to add a CSV (combined_zipcodes.csv) that merges a larger ZIP csv (us-zip-code-latitude-and-longitude.csv) with the existing one. This covers many more zips that are in the PHZ data but didn't have location info before.
2. The refactor of the code is pretty significant, and wholly unnecessary, but was primarily just a motivating case to experiment in my own Python learning.

[waldoj]

Thank you, Phil! I'm going to get this to build (I seem to have some configuration things wrong) before I merge it. I only did a 5-minute review, but this all seems like a big improvement. :)

[waldoj]

...or merge and then build because life is short and the risk is low with an API made out of static files. :)