Adjust stream constraints in options (resolution etc)

[RuneSP]

It would be nice to be able to change the default stream contraints when creating the scanner, or maybe allow user to pass a custom contstraint set.

I've currently solved it be overwriting the _getCameraStream method just after creating the scanner.

I'm capturing an image from the video stream when the user scans a barcode (as our labels contains manually written notes, its nice to save them digitally), but the default resolution is too low.