feat(优化): sourcemap方便DEBUG，build前清空

Author: licoded

Diff for: webpack.config.js

@@ -7,7 +7,9 @@ module.exports = {
   output: {
     filename: 'main.js',
     path: path.resolve(__dirname, 'dist'),
+    clean: true,
   },
+  devtool: 'inline-source-map',
   devServer: {
     static: './dist',
   },