Stream sane_read

[Jupi007]

Stream sane_read() response instead of needing to call this method repetitively.

[Craftplacer]

I've been asking myself how much "cleaner" we should present the developer with options.

Should be there a Stream<Uint8List> Sane.scan(Function(Options options) onConfigure) method instead?

If yes, should we still expose start() set_io_mode() cancel() to the developer?

[Jupi007]

Imo, this and #18 should be part of an higher level scanner.dart library.
In sane.dart we expose the low level pure dart implementation, on top of what we can build something with a better oop approach.
sane.dart should stay as simple and as close as possible to the C library, so we can test it easily.