Add method for accessing url query string params

[chrismytton]

Problem

Often in scrapers you need to pull information out of the querystring, which requires the following boilerplate code:

def query
  @query ||= URI.decode_www_form(URI.parse(url).query).to_h
end

Proposed solution

It would be good to offer some equivalent of the above code for the common case where we want to pull information out of the query string.

[chrismytton]

As @tmtmtmtm mentioned in everypolitician-scrapers/spain_congreso_es#14 (comment) perhaps query_string would be a better name for this.