Add didGeneratePNGData callback prop

[andrewrothman]

Hi!

I've been using qrcode.react at my employer, and we needed to have a download button to save the generated QR code as an image.

How we're using this PR internally:

<QRCode didGeneratePNGData={qrcodeData => this.setState({ qrcodeData })} />

state = {
    qrcodeData: ""
};

onDownloadClicked() {
    const downloadAnchor = document.createElement("a");
    downloadAnchor.setAttribute("download", "QR.png");
    downloadAnchor.setAttribute("href", this.state.qrcodeData);
    downloadAnchor.click();
}

This has been working perfectly for us for ~6 moths. I just updated it to the newest zpao/master for React 16, and tested it within our project.

Let me know what you think.

Best,
Andrew

[Domino987]

Any news on this?