Trainbird

I just had a look into Babel 7 and it looks as if there was an async way of transforming now. See the compat note in the [Babel Docs](https://babeljs.io/docs/en/babel-core/#transform) for...

[Maybe](https://github.com/airbnb/babel-plugin-inline-react-svg/issues/57#issuecomment-472094021) Babel 7 solves the issue ... this PR in its original idea (synchronizing Promises with a 3rd party plugin) is ([understandably](https://github.com/airbnb/babel-plugin-inline-react-svg/pull/35#discussion_r197342099)) not compliant with package policy.