Add better highlighting for show-paren mode

show-paren-mode includes two faces show-paren-match and
show-paren-mismatch.

With this commit, show-paren-mismatch is highlighted with a red
background and black foreground while show-paren-match is highlighted
using uses a dark gray20 background either on the text or throughout
the body depending on whether show-paren-style is parenthesis,
expression or mixed.