Suggestion: Allow custom diftools

[singiamtel]

It'd be nice to be able to use custom difftools to render the diff, like https://github.com/Wilfred/difftastic

This is how I'd do the same thing with git, for example:

# .gitconfig
[diff]
	tool = difftastic
[difftool "difftastic"]
    cmd = difft "$LOCAL" "$REMOTE"