Prepare to PostCSS v3

Some changes have to be done to match [PostCSS 3](https://github.com/postcss/postcss/releases/tag/3.0.0).

You have to review this PR, I'm not sure everything is OK.
For example, 2 tests failed if it misses a semicolon at the end and I don't know why. Plus sourcemaps creation has changed.

But it's a start...