Add command cruft difftool

[souliane]

Hi there,

thanks for this project!

I like the concept a lot, but I think that a command to display the diff using the configured git difftool would be great!
If I am not mistaken, the current cruft diff relies on git and how the user configured it to display the diff. The idea is to do the same with git difftool.
For instance, my ~/.config/git/config is setup to open Meld when I type git difftool, and cruft difftool could do the same.

PS: then we can even think of cruft mergetool :-)