React Native Mac support

[marcosrdz]

Using this package with React Native Mac (https://microsoft.github.io/react-native-windows/) results in:

null is not an object (evaluating RNRandomBytes.seed)

[unixvb]

The same for react-native-macos
TypeError: null is not an object (evaluating 'RNRandomBytes.seed')

[Dax911]

react-native-bip39 also relies on this and I am still getting the TypeError