Update clean-css to v2.0.7 / Add test for clean-css options

* `require('clean-css').process` is removed at the release of v2.0.0. Instead, now `require('clean-css')` is a constructor of CleanCSS object and it can be used as `new CleanCSS([options]).minify`.
* I added a test for [clean-css options](https://github.com/GoalSmashers/clean-css#how-to-use-clean-css-programmatically).