CSS: support add newline between rules

It's suggested by
[Google](https://google-styleguide.googlecode.com/svn/trunk/htmlcssguide.xml#CSS_Formatting_Rules)
that newline should be added between rules, so I think we should support
that and try to make it.

I added this behavior to both JS and Python with tests. Besides, I added
the new option to README and cli.js.

If I need to modify other files, please let me know.

Thanks!