add qrcode data url render callback

[JackZhang1988]

QRCode data render async, so we need a callback function to notice the QRCode had rendered

[ReinheimerPiano]

My congratulations for the initiative, I'm going through the same problem, I implemented on my own account I would like to know how I would use this success event in my code, I tried this way but I still could not: qrcode.success (function {
appreciate!