Add babel-plugin-transform-react-router-optimize

[ZauberNerd]

This transform will help to reduce file size in projects which use the react-router.

The react-router guides mention how to import only neccessary parts of the library to keep the bundle size small: https://github.com/reactjs/react-router/blob/master/docs/guides/MinimizingBundleSize.md

This babel plugin will do the transformation automatically.

[NeXTs]

@thejameskyle Is this repo still maintained?