Fix CSS source map issue

[yinanfang]

• Commit: 38714afd922dcafb3470c54f449d72fb6c0134e6
  • [x] Add sourceMap after postcss-loader: https://github.com/postcss/postcss-loader#user-content-sourcemaps
  • [x] No ExtractTextPlugin for dev version: https://ihaveabackup.net/article/sass-with-sourcemaps-webpack-and-live-reload
  • [x] Baisic setup: https://github.com/webpack/css-loader/issues/228#issuecomment-182233074
• Commit: ebd3a2075acb9923fbb8afcaed2ca01e0a9c7e4e
  • [x] No source map for production