title: why module build error happens when run webpack?
body: <p>I'm writting a loader while this error happens:</p>
  <pre><code>ERROR in ./src/components/notFound.vue
  Module build failed (from ./loader/cssExchangeLoader.js):
  /Users/laiyinan/Project/前端开发/blog/loader/cssExchangeLoader.js:3
  export default function(source){
  ^^^^^^
  
  SyntaxError: Unexpected token export
  </code></pre>
  <p>loader(ALL):</p>
  <pre><code>var loaderUtils = require('loader-utils');
  
  export default function(source){
      let options = loaderUtils.getOptions(this);
      console.log(options);
      console.log(source.substr(0,10));
      return `export default ${JSON.stringify(source)}`;
  } 
  </code></pre>
  <p>webpack.config(Part of):</p>
  <pre><code>{
                  test: /(\.css$)|(\.scss$)|(\.vue$)/,
                  use: [
                      {
                          loader: path.resolve(root,'loader/cssExchangeLoader.js'),
                          options: {
                              target: 'red',
                              alternative: 'green'
                          }
                      }
                  ],
                  exclude: [/node_modules/,/loader/]
              }
  </code></pre>
  <p>why module build error happens when run webpack? And how to fix it? Thanks in advance!</p>