Support external tool for adding color and syntax highlighting to diffs

Hi @altsem, gitu looks fantastic. Would it be possible to support using an external tool to add colors to diffs before display? I'm the author of https://github.com/dandavison/delta, and I'd love to be able to use gitu with colors supplied by delta. Delta works by taking diff format input on stdin, and writing the same diff to stdout with ANSI color escape sequences inserted. It has an option --color-only that guarantees not to make any structural alterations to the diff, other than adding colors (syntax highlighting and/or background colors). So the simplest implementation would be to allow users to configure an external binary that is passed the raw diff as text, just before gitu is ready to display it.

(Incidentally, delta can be used in this way to add syntax highlighting and colors to emacs magit, but of course the implementation there is more complex because the ANSI color escapes produced by delta have to be translated to the way emacs represents colored text: https://github.com/dandavison/magit-delta.)

No file or test is named in the issue. Start by tracing gitu's diff-display path and configuration handling, then examine how an external command could receive raw diff text and return ANSI-colored output, using delta's --color-only behavior as the contract. Done means a configured tool colors displayed diffs without structural changes.