add a plugin option to support passing options cssnano v4 (#73)

* pass cssNanoOptions to cssnano

* update doc for cssProcessorPluginOptions

* change cssNanoOptions to cssProcessorPluginOptions

README.md

@@ -25,6 +25,7 @@ The plugin can receive the following options (all of them are optional):
 * assetNameRegExp: A regular expression that indicates the names of the assets that should be optimized \ minimized. The regular expression provided is run against the filenames of the files exported by the ExtractTextPlugin instances in your configuration, not the filenames of your source CSS files. Defaults to `/\.css$/g`
 * cssProcessor: The CSS processor used to optimize \ minimize the CSS, defaults to [cssnano](http://github.com/ben-eb/cssnano). This should be a function that follows cssnano.process interface (receives a CSS and options parameters and returns a Promise).
 * cssProcessorOptions: The options passed to the cssProcessor, defaults to `{}`
+* cssProcessorPluginOptions: The plugin options passed to the cssProcessor, defaults to `{}`
 * canPrint: A boolean indicating if the plugin can print messages to the console, defaults to `true`
 
 ## Example:
@@ -45,7 +46,9 @@ module.exports = {
     new OptimizeCssAssetsPlugin({
       assetNameRegExp: /\.optimize\.css$/g,
       cssProcessor: require('cssnano'),
-      cssProcessorOptions: { discardComments: { removeAll: true } },
+      cssProcessorPluginOptions: {
+        preset: ['default', { discardComments: { removeAll: true } }],
+      },
       canPrint: true
     })
   ]

src/index.js

@@ -23,6 +23,9 @@ class OptimizeCssAssetsWebpackPlugin extends LastCallWebpackPlugin {
     this.options.cssProcessorOptions = !options || options.cssProcessorOptions === undefined ?
       {} :
       options.cssProcessorOptions;
+    this.options.cssProcessorPluginOptions = !options || options.cssProcessorPluginOptions === undefined ?
+      {} :
+      options.cssProcessorPluginOptions;
   }
 
   buildPluginDescriptor() {
@@ -59,7 +62,7 @@ class OptimizeCssAssetsWebpackPlugin extends LastCallWebpackPlugin {
       }
     }
     return this.options
-      .cssProcessor.process(css.source, processOptions)
+      .cssProcessor.process(css.source, processOptions, this.options.cssProcessorPluginOptions)
       .then(r => {
         if (processOptions.map && r.map && r.map.toString) {
           assets.setAsset(assetName + '.map', r.map.toString());