[resolver-webpack] Compat with multiple configs

`webpack.config.js` can be an array of webpack config objects like so:

``` js
module.exports = [
  {
    name: 'configurator',
  },
  {
    name: 'scene',
  }
];
```

Currently, I have to append this to my config file:

``` js
module.exports.resolve = {
  extensions: [
    '',
    '.jsx',
    '.js',
  ],
};
```

... which shouldn't be necessary. Please add support for multiple webpack configs.
