enablePostCssLoader(postCssLoaderOptionsCallback = () => {}) {
        webpackConfig.enablePostCssLoader(postCssLoaderOptionsCallback);

        return this;
    }
Enables the postcss-loader

Once enabled, you must have a postcss.config.js config file.

https://github.com/postcss/postcss-loader

```
Encore.enablePostCssLoader();
```

Or pass options to the loader

```
Encore.enablePostCssLoader(function(options) {
    // https://github.com/postcss/postcss-loader#options
    // options.config = {...}
})
```

@param {OptionsCallback<object>} postCssLoaderOptionsCallback
@returns {Encore}