Update webpack.config.js and package.json to support PostCSS

[oleq]

diff --git a/package.json b/package.json
index 18261989..d24ae539 100644
--- a/package.json
+++ b/package.json
@@ -30,16 +30,14 @@
     "@ckeditor/ckeditor5-utils": "^1.0.0-alpha.2"
   },
   "devDependencies": {
-    "@ckeditor/ckeditor5-dev-utils": "^5.0.0",
-    "@ckeditor/ckeditor5-dev-webpack-plugin": "^2.0.22",
+    "@ckeditor/ckeditor5-dev-utils": "^7.0.3",
+    "@ckeditor/ckeditor5-dev-webpack-plugin": "^3.0.4",
     "babel-minify-webpack-plugin": "^0.2.0",
-    "css-loader": "^0.28.7",
-    "extract-text-webpack-plugin": "^3.0.0",
-    "node-sass": "^4.5.3",
+    "extract-text-webpack-plugin": "^3.0.2",
     "raw-loader": "^0.5.1",
-    "sass-loader": "^6.0.6",
-    "style-loader": "^0.18.2",
-    "webpack": "^3.6.0",
+    "postcss-loader": "^2.0.10",
+    "style-loader": "^0.19.1",
+    "webpack": "^3.10.0",
     "webpack-sources": "1.0.1"
   }
 }

diff --git a/webpack.config.js b/webpack.config.js
index 0a258ef2..c1e24a14 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -7,6 +7,7 @@
 const path = require( 'path' );
 const webpack = require( 'webpack' );
 const { bundler } = require( '@ckeditor/ckeditor5-dev-utils' );
+const { getPostCssConfig } = require( '@ckeditor/ckeditor5-dev-utils' ).styles;

 const BabelMinifyPlugin = require( 'babel-minify-webpack-plugin' );
 const ExtractTextPlugin = require( 'extract-text-webpack-plugin' );
@@ -30,19 +31,24 @@ module.exports = {
                                     use: [ 'raw-loader' ]
                            },
                            {
-                                    test: /\.scss$/,
-                                    use: ExtractTextPlugin.extract( {
-                                             fallback: 'style-loader',
-                                             use: [
-                                                      {
-                                                               loader: 'css-loader',
-                                                               options: {
-                                                                        minimize: true
-                                                               }
-                                                      },
-                                                      'sass-loader'
-                                             ]
-                                    } )
+                                    test: /\.css$/,
+                                    use: [
+                                             {
+                                                      loader: 'style-loader',
+                                                      options: {
+                                                               singleton: true
+                                                      }
+                                             },
+                                             {
+                                                      loader: 'postcss-loader',
+                                                      options: getPostCssConfig( {
+                                                               themeImporter: {
+                                                                        themePath: require.resolve( '@ckeditor/ckeditor5-theme-lark' )
+                                                               },
+                                                               minify: true
+                                                      } )
+                                             },
+                                    ]
                            }
                   ]
          },