Add OpenAddresses support

Hooray! OpenAddreses support! This PR references #7 and has:

* README info about deploying with OpenAddresses CSVs
* changes to `app.py` to remove `parcelid` from output geojson
* changes to `index_addresses.py` to allow passing CSVs to app through command line, add more than one CSV at a time, and index based on OA schema

This PR doesn't have:

* changes to the UI
* changes to the map (@livienyin maybe you can help here?)