Add parameter disableStructuralMinification

Adds the ability to use the disableStructuralMinification parameter when calling css-loader:

```js
module.exports = {
  module: {
    loaders: [
      { test: /\.css/, loader: "style-loader!css-loader?disableStructuralMinification=true" },
      { test: /\.png/, loader: "url-loader?limit=100000&mimetype=image/png" },
      { test: /\.jpg/, loader: "file-loader" }
    ]
  }
};
```
lf disableStructuralMinification is true, csso will not do structural magnification which is destructive, in some cases breaking @keyframe rules.