🐛 props fix

[shazha]

This PR addresses the following issues:

1. prop autoColor and whiteMargin are always TRUE and it's caused by the following code in vue-qr.js:
   autoColor: toBoolean(that.autoColor) || true
whiteMargin: toBoolean(that.whiteMargin) || true
2. prop logoCornerRadius not working with number value 0 while it's working when using string value 0 due to the code:
   logoCornerRadius: that.logoCornerRadius || 8
3. In awesome-qr, the bindElement is optional. It provides the ability to not bind to any HTML node and only return the dataURI generated for further processing. However, vue-qr always binds the generated QR image data to a HTML node and it's not so handy in some scenarios.

To adress issue 1 and issue 2, I moved the default value for each prop to props definition and added validations for them. And for backward compatibility, String value is also allowed for whiteMargin, autoColor and binarize.
To address issue 3, I added a new prop named bindElement which accepts Boolean value with default value TRUE.

Regards,

[Binaryify]

you are awesome, I appreciate it