Improve `css` method

Bring Cheerio's `css` method closer to jQuery's. Specifically:

- Implement new signatures
  - Setting via a function
  - Getting via an array
- Implement API for unsetting styles (via the empty string)
- Fix a bug when setting multiple elements' styles at once
- Remove unspecified 0-argument signature

Additionally, document the method in the project's `Readme.md` file