bug#28047: 25.2; Add another show-paren face

[Eli Zaretskii]

> From: Felipe Ochoa <felipe.nospam.ochoa@gmail.com>
> Date: Fri, 11 Aug 2017 15:47:39 +0200
> 
> I use show-paren-style in mixed mode, and would like to be able to use
> different faces depending on whether the match is being displayed in
> 'expression style or 'parenthesis (I want a stronger contrast in
> parenthesis than in expression). The attached patch splits the
> show-paren-match face into two, show-paren-match-expression and
> show-paren-match, using inheritance to make the split styling opt-in.
> 
> With this change, users can style the two faces separately if they wish.

Thanks, pushed to master.