Fill in blanks in contacts yaml using XLS file

~~Branched off of #30's code -- please merge that first~~

This adds a script (`layer_with_csv.py`, based on the ideas in https://github.com/18F/foia-hub/blob/master/foia_hub/scripts/load_agency_contacts.py), which will fetch the xls file of additional contact information. It then runs through all of the YAML files in the `data` directory and looks for matching departments/agencies. If a row in the xls file contains information not in the YAML, the YAML file is updated.

Instead of separating name, phone, etc. as present in the XLS, contact information is concatenated into a string to match the YAML structures (e.g. `foia_officer: "John Smith, Phone: (111) 222-3333").