Move to PostCSS

[ai]

Right now Stylestats uses Rework CSS parser. But PostCSS CSS arser may be better:

1. It is 1.4 times faster.
2. It is much stable. For example, we uses integration tests with GitHub, Twitter and BrowserHacks CSS.
3. It has different parsers too. Like postcss-safe-parser for legacy CSS with syntax errors or postcss-scss.
4. It has bigger user base, so you will have better support and faster fixes.