因为.vue的引入没有后缀名，自己加上

module.exports = {
  entry: './src/main.js',
  output: {
    filename: "./js/bundle.js"
  },
  resolve: {
      alias: {
        'vue$': 'vue/dist/vue.js' // 用 webpack 1 时需用 'vue/dist/vue.common.js'
      },
      extensions: ['*', '.js', '.vue']
}
}