def lookup_tag_due_date
      config.fetch(:tags).fetch(match[:tag])
    rescue KeyError
      msg = "#{match[:tag]} tag not defined in config file"
      raise KeyError, msg
    end
def lookup_tag_due_date
      config.fetch(:tags).fetch(match[:tag])
    rescue KeyError
      msg = "#{match[:tag]} tag not defined in config file"
      raise KeyError, msg
    end
A tag was referenced, so let's see when that's due
 @return [DueDate]
 @raise [KeyError] if the tag does not reference a due date in the config
 @api private