code: func Deferred(key string, f func() string) Loggable {
  	function := func() map[string]interface{} {
  		return map[string]interface{}{
  			key: f(),
  		}
  	}
  	return LoggableF(function)
  }
docs: // Deferred returns a LoggableF where the execution of the
  // provided function is deferred.