Update sites-extended.csv to capture agency abbreviation

[h-m-f-t]

On analytics.usa.gov, sites-extended.csv is published nightly, which is a "dataset of federal hostnames for which DAP observed at least one recorded visit in the previous 14 days" (source: Pulse).

The README for this repo encourages agencies to enter their org's abbreviation as a tag in the DAP code.

It would be really helpful for sites-extended.csv to include the agency abbreviation e.g., in another column, enabling attribution for those instances when a domain's name itself isn't sufficient.

cc: @jsf9k @climber-girl

[tdlowden]

Will see what we can do!