Use peerDependencies in package.json instead of dependencies

References:

* [How to write a loader](http://webpack.github.io/docs/how-to-write-a-loader.html#use-a-library-as-peerdependencies-when-they-wrap-it) (webpack docs)
* [Peer dependencies](http://blog.nodejs.org/2013/02/07/peer-dependencies/) (node.js blog)