Add configuration file to specify defaults

Maybe something like [l10n.yml](https://docs.flutter.dev/ui/accessibility-and-internationalization/internationalization#configuring-the-l10n-yaml-file) in flutter:

`.yarb.yml`
```
arb-dir: l10n
template-arb-file: app_en.arb
use-escaping: true
android:
   output-path: app/src/main/res
ios:
  output-path: AppIOS/Resources/{lang}.lproj
```