(1) rules that we enforce.
(2) rules that we would like to enforce, but haven't cleaned up the codebase to
    turn on yet (or we need to make the scalastyle rule more configurable).
(3) rules that we don't want to enforce.

Author: Reynold Xin <rxin@databricks.com>

Closes #6543 from rxin/scalastyle and squashes the following commits:

beefaab [Reynold Xin] [SPARK-7986] Split scalastyle config into 3 sections.