Add tests for downlevel-revealed IE cond comments
Implement downlevel-revealed IE cond comments

Enable creation of "downlevel-revealed" IE conditional comments:

    /![if !IE]
      A comment

Will produce

    <!--[if !IE]><!--> A comment <!--<![endif]-->

or if :conditional_comments_valid_html is false:

    <![if !IE]> A comment <![endif]>
Don't generate MS-style conditional comments

Remove option to allow generating MS-style conditional comments, only
generate the valid HTML style.
Document downlevel-revealed conditional comments
Merge branch 'conditional-comments-bang-ie'