Difftastic support

[HRKings]

I use delta most of the time, but difftastic is great to have sometimes to get a "more precise" diff.
I've forked the repo and added support for it with a subcommand.

The main problem with difftastic is it's calling method, it must be called with git's difftool; and the speed, sometimes if you have a bunch of files it takes a while to process everything. For the fork I added support for streaming the chunks that come from the command running in the background and a loading spinner to show the streaming status and the diff loading.
Also when in watch-mode, I changed the caching to only reprocess the changed files to avoid a root recalculation of diff.

I understand that those are pretty big changes and maybe they aren't fit for the repo, that's the main motivation behind the discussion.