Flutter Plugin Prototype

Hello! I've been making some progress in implementing a [Flutter plugin for Vosk](https://github.com/ethan-ou/offline_speech_recognition). It currently only binds to the Android library, but it should be possible to bind to the iOS side as well.

There's also an [example Android apk](https://github.com/ethan-ou/offline_speech_recognition/releases/tag/v0.0.1) to try the plugin on your phone. The example app downloads the model from the alphacep servers then unzips it into the assets directory for usage. It's more complicated but it means every current language supported by Vosk can be used.

I've kept the API relatively low-level, but I'm no expert in Java or Flutter so some of the design might be off. On the Flutter side, the API is as so:

```dart
OfflineSpeechRecognition.load(String path);
OfflineSpeechRecognition.start();
OfflineSpeechRecognition.stop();
OfflineSpeechRecognition.destroy();
OfflineSpeechRecognition.onResult();
OfflineSpeechRecognition.onPartial();
```

If you think this is a good project to add to alphacep, I'm also open to transferring ownership if that's something you want to do.