Feature Request - ability to invoke Fork's diff view on files via fork_cli

[JonathanDoughty]

This would support using Fork's diff view for more general purpose diff navigation invoked from other, external tools.

Ideally one could add something like:

[difftool "fork"]
cmd = /Applications/Fork.app/Contents/Resources/fork_cli "$LOCAL" "$REMOTE"

(or just /usr/local/bin/fork if one has installed the command line CLI helper) to one's ~/.gitconfig to do a git diff ... or use other tools that need a diff viewer. Perhaps a new diff CLI command would be required so that the above would need to be `...fork_cli "diff" "$LOCAL" "$REMOTE"