code: func (otp *HOTP) QR(label string) ([]byte, error) {
  	u := otp.URL(label)
  	code, err := qr.Encode(u, qr.Q)
  	if err != nil {
  		return nil, err
  	}
  	return code.PNG(), nil
  }
docs: // QR generates a byte slice containing the a QR code encoded as a
  // PNG with level Q error correction.