hotp.go
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
}
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
}
// QR generates a byte slice containing the a QR code encoded as a
// PNG with level Q error correction.