* CSS and Map is casted as Buffer from binding.
Advantages:
  * Buffers are super fast.
  * User can write them to file as is.
  * User can forward to response stream as is.
  * User can strigify it as is:
    * `result.css.toString()`
    * `result.map.toString()` or
      `JSON.stringify(result.map)`
* Updates tests and CLI usage accordingly.
* Updates README to reflect the changes.

Issue URL: #711.
PR URL: #729.