code: def replace_variables(txt, original = nil)
        if ! original.nil? && txt.include?("#USER#")
          username = tweet_user(original)
          txt.gsub("#USER#", username)
        else
          txt
        end
      end
docs: do some simple variable substitution.  for now, it only handles
   replacing #USER# with the screen of the incoming tweet, but it
   could do more if needed