Sourcemap Support

[boogerlad]

again, to achieve feature parity with clean-css

[yisibl]

We will get free source map support if use PostCSS. :ghost:

推荐使用 PostCSS，获得更好的 source map 支持。

https://github.com/postcss/postcss#source-map-1

[army8735]

最近没啥时间，我记下来慢慢看。sourcemap这方面确实没啥研究，进展会比较慢。