title: Add autoprefixer in webpack
body: <p>My webpack.config.js file:</p>
  
  <pre><code>var path = require("path"),
      src = path.resolve(__dirname, "src"),
      dist = path.resolve(__dirname, "dist"),
      webpack = require("webpack");
  
  var HtmlWebpackPlugin = require("html-webpack-plugin");
  var ExtractPlugin = require("extract-text-webpack-plugin"),
      CleanWebpackPlugin = require("clean-webpack-plugin"),
      CopyWebpackPlugin = require("copy-webpack-plugin");
  
  var extractCssPlugin = new ExtractPlugin({
    filename: "css/main.min.css"
  });
  
  var extractHtmlPlugin = new ExtractPlugin({
    filename: "[name].html"
  });
  
  module.exports = {
    entry: {
      bundle: src + "/app/app.js"
    },
    output: {
      path: dist,
      filename: 'js/[name].min.js'
    },
    devtool: 'source-map',
    module: {
      rules: [
        {
                  test: /\.js?/,
                  include: src,
                  loader: "babel-loader",
                  query:{
                      presets: ["react", "es2015", "stage-2"]
                  }
              },
        {
            test: /\.pug$/,
            loaders: ['file-loader?name=[name].html', 'pug-html-loader?pretty&amp;exports=false']
        },
        {
          test: /\.scss$/,
          use: extractCssPlugin.extract({
            use:[
              {
                loader: "css-loader",
                options: {sourceMap: true, minimize: true}
              },
              {
                loader: "sass-loader",
                options: {sourceMap: true}
              },
              "postcss-loader"
            ]
          })
        }
      ]
    },
  
    plugins: [
      cleanPlugin,
      copyPlugin,
      extractCssPlugin,
      extractHtmlPlugin,
      new webpack.DefinePlugin({
        'process.env': {
          'NODE_ENV': JSON.stringify('production')
        }
      })
    ]
  }
  </code></pre>
  
  <p>My postcss.config.js inside src/css/postcss.config.js</p>
  
  <pre><code>module.exports = {
    plugins: [require("autoprefixer")]
  };
  </code></pre>
  
  <p>package.json</p>
  
  <pre><code>{
    "name": "test_project",
    "version": "1.0.0",
    "description": "",
    "scripts": {
      "test": "echo \"Error: no test specified\" &amp;&amp; exit 1",
      "build:prod": "webpack -p",
      "build:dev": "webpack -d"
    },
    "browserslist": ["last 2 versions"],
    "keywords": [],
    "author": "",
    "license": "ISC",
    "devDependencies": {
      "autoprefixer": "^8.0.0",
      "babel-core": "^6.26.0",
      "babel-loader": "^7.1.2",
      "babel-preset-es2015": "^6.24.1",
      "babel-preset-react": "^6.24.1",
      "babel-preset-stage-2": "^6.24.1",
      "clean-webpack-plugin": "^0.1.17",
      "copy-webpack-plugin": "^4.3.1",
      "css-loader": "^0.28.9",
      "cssnano": "^3.10.0",
      "extract-loader": "^2.0.1",
      "extract-text-webpack-plugin": "^3.0.2",
      "file-loader": "^1.1.6",
      "html-webpack-plugin": "^2.30.1",
      "node-sass": "^4.7.2",
      "postcss-cssnext": "^3.1.0",
      "postcss-loader": "^2.1.0",
      "pug-html-loader": "^1.1.5",
      "react": "^16.2.0",
      "react-dom": "^16.2.0",
      "react-router": "^4.2.0",
      "react-router-dom": "^4.2.2",
      "sass-loader": "^6.0.6",
      "style-loader": "^0.19.1",
      "webpack": "^3.10.0",
      "webpack-dev-server": "^2.11.0"
    },
    "dependencies": {
      "postcss-import": "^11.1.0",
      "precss": "^3.1.2"
    }
  }
  </code></pre>
  
  <p>Just need to add autoprefixer to my css code once the main.scss is compiled to main.css in webpack. I did try using the post-css loader, but I couldn't get it right. </p>