Support EventChannel(s)

[larssn]

Recommend supporting Streams through EventChannel, as it makes some operations easier (like having a listener running in the background).

Then it would be possible to having a simple API like:

var streamSub = FlutterNfcKit.tags.listen((tag) {
   print(tag);
});

Just a suggestion. Sadly I have no time to do a PR.

[Harry-Chen]

This is possible, but will become a total different set of APIs. Maybe the foreground dispatch mode mentioned in #17 can work together with this Stream-like API.