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!

complete cli.js

fix bugs in test.py

fix tests

use newLine function

fix python tests

fix tests

use indentLevel instead of nestedLevel

match default js tests

still

Revert "still"

This reverts commit 32797e83f07541c248ebe7fe9926e8a754556c98.

Find it!

update tests file

add more tests

add test