Writing Exif Data to in-memory buffer

Hello,

I was wondering if it would be possible to write exif data directly to an in-memory buffer, instead of having to write the file (even temporarily) to storage. Something like 

```dart
final exif = await Exif.fromBytes(imageBytes);
...
final newImage = await exif.getImageBytes();
await exif.close(); 
```

would be perfect.