code: func ClientCert(certificate, key string) Option {
  	return func(c *CredHub) error {
  		cert, err := tls.LoadX509KeyPair(certificate, key)
  		if err != nil {
  			return err
  		}
  		c.clientCertificate = &cert
  
  		return nil
  	}
  }
docs: // ClientCert will use a certificate for authentication