Support for inverted barcodes

[alextarana]

Hi, thanks for this plugin.

It would be awesome if it could manage to scan an inverted barcode (white bars on a black or kinda background).

I think that it could be done in some steps:

1. the plugin should try to detect the barcode
2. if the detection fails, the plugin should try to invert the data source (image or stream from the camera)
3. the plugin should retry to detect the barcode
4. if it worked, return the result