code: func (c *Config) Validate() error {
  	if c.Output == nil {
  		return errors.New("butler: an Output must be supplied")
  	}
  	if err := c.Project.Validate(); err != nil {
  		return fmt.Errorf("invalid project: %v", err)
  	}
  	if err := c.Prefix.Validate(); err != nil {
  		return fmt.Errorf("invalid prefix: %v", err)
  	}
  	return nil
  }
docs: // Validate validates that the configuration is sufficient to instantiate a
  // Butler instance.