Support diff-hl

[darkstego]

The diff-hl mode shows the changes in code in the fringe. Using moe-dark the changes are colors are very hard to notice. I was wondering if those could be supported in the theme.

This is what I am using with moe-dark to make them more visible
(diff-hl-insert ((,class (:background ,green-4 :foreground ,green-2))))
(diff-hl-change ((,class (:background ,blue-3 :foreground ,blue-2))))
(diff-hl-delete ((,class (:background ,red-3 :foreground ,red-1))))

[nyqvist]

Maybe the diff-hl-change should be:
(diff-hl-change ((,class (:background ,blue-3 :foreground ,blue-1))))

To make the '!' more visible