Webpack 2 Support

Hi, thanks for this plugin! I haven't tried it with webpack 1 but at least with webpack 2 I can't use it. My conf:

```
new ExternalsPlugin({
type: 'amd',
include: process.cwd() + '/bower_components'
}),
```

```
10% building modules
Fatal error: Cannot read property 'request' of undefined
```

https://github.com/Morhaus/webpack-externals-plugin/blob/master/index.js#L19 seems to be it. Time to release a webpack 2 version of the plugin?