Add document boundaries on preview (camera view)

[psalkowski]

It's really nice library that is also used in your react-native lib, but when compares to iOS, the only issue that we have is lack of preview for document boundaries. User do not know if the document has been found or not and need to capture image and then the crop can be set correctly, or totally not found and need to set it manually.

For improving user experience, it would be nice to see document boundaries in real-time on camera view