Create `authority` scrape parameter for websites that serve multiple authorities

[adrianshort]

eg Northern Ireland:

http://epicpublic.planningni.gov.uk/publicaccess/search.do?action=advanced&searchType=Application

which is an Idox site that has a param called searchCriteria.localGovernmentDistrict.

An alternative approach would be to list each authority separately in authorities.csv and add extra scrape parameters to that file so that only the specific authority is searched.

See also #17.