[SPARK-13629][ML] Add binary toggle Param to CountVectorizer

## What changes were proposed in this pull request?

This is a continued work for https://github.com/apache/spark/pull/11536#issuecomment-198511013,
containing some comment update and style adjustment.
jkbradley

## How was this patch tested?

unit tests.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #11830 from hhbyyh/cvToggle.