Support platform-specific asset configs in react-native.config.js

[roryabraham]

Fixes https://github.com/unimonkiez/react-native-asset/issues/34

First-time contributor to this repo, so I'm definitely open to feedback

[tobyt42]

This change would be great to get merged and released. It works as expected in our project.

[RichardFevrier]

@unimonkiez would you mind looking at the different proposed pull requests for your project ❤️

[unimonkiez]

Available on v2.1.0

[RichardFevrier]

Thank you a lot 🙏