* In order to skip the importer, user must
  return `sass.types.NULL` (or the shorter
  alias `sass.NULL`).
* See the added test on usage.
* Backward compatible: part of me want still
  wants to make it non-backward compatible
  because:
  * We can: in major version v3.0.
  * It will keep the API clean.
  * It will keep the docs clear: type of
    `options.importers` is array of functions.
* Updates docs.

Issue URL: #821.
PR URL: #832.