New option: g:signify_difftool

Some VCS rely on an external diff tool, because they don't support the
-U0 flag natively. Most of the time that's the standard 'diff' utility.

But fritzophrenic was reporting that even a GNU diff won't always be
installed as 'diff', but e.g. as 'gnudiff' instead.

You can set the external diff tool now.

Closes #29.