def retrieve_asset(uri)
      uri = URI.join(full_source_uri, uri)
      cache(uri) do
        case
        when %w[http https].include?(uri.scheme)
          Net::HTTP.get(uri)
        when uri.scheme.eql?('file')
          File.read(uri.path)
        end
      end
    end
def retrieve_asset(uri)
      uri = URI.join(full_source_uri, uri)
      cache(uri) do
        case
        when %w[http https].include?(uri.scheme)
          Net::HTTP.get(uri)
        when uri.scheme.eql?('file')
          File.read(uri.path)
        end
      end
    end
source_uri: location of the original document, used to retrieve relative URI's
 asset_dir: location where assets will be stored
 destination: file that will be generated, used to create relative URI's from