Support alternative reading libraries

[rmtmckenzie]

Right now, this plugin handles the 'difficult' part of getting images from the camera and hooking it into the processing part (which can't quite be done just using the Flutter camera library unfortunately). It currently uses FirebaseMobileVision's detector.

What I'd like to propose if there's enough interest, is to split the plugin into a few different parts.

1. qr_mobile_vision
2. qmv_backend_android_firebase
3. qmv_backend_ios_firebase
4. qmv_backend_ios_native
5. qmv_backend_android_zxing?
   etc

You'd then have to depend on the base library and then pass in which backend you wanted to use. This would allow greater flexibility at the cost of a bit of complexity.

Unfortunately, some fairly major refactoring would be needed on the native side of things, and realistically the dart API would change pretty significantly as well. This would be a decent-sized undertaking, so I won't get to it right away and even then only if there's interest shown in it.