Allow to set custom lookup option for Geocoder.search, ...

... Geocoder::Query.new and ActiveRecord::Base.(reverse_)geocoded_by

  Examples:

    class City < ActiveRecord::Base
      geocoded_by :address, :lookup => :nominatim
    end

    class City < ActiveRecord::Base
      geocoded_by :address, :lookup => lambda{|obj| obj.custom_lookup }

      def custom_lookup
        # return custom lookup for record
      end
    end

    Geocoder.search("address", :lookup => :nominatim)