Add colordiff (standalone Perl program)

From: https://raw.github.com/daveewart/colordiff/v1.0.13/colordiff.pl
Add diff-highlight (standalone Perl program)

From:
https://raw.github.com/git/git/master/contrib/diff-highlight/diff-highlight
Add mailcap config

Respected by Alpine.  Utilizes colordiff and diff-highlight.