Would it be possible to include an option to show damage done versus Undead mobs?

With the inclusion of items like https://classic.wowhead.com/item=23207/mark-of-the-champion and mages running the Undead Cleansing Set (https://classic.wowhead.com/gear-planner/mage/gnome/AjwAgVNjXmQCVGiDS6pfZYVaLU45BkrAh1SsXmSIU2A2QolaM04oilosYfYLVM0MSssNWqcOS7MPTZEQUxkSLeQ) I think it would make a great addition if the +damage would be visible on the character pane.

This could either be on its own, or included in the total Spell Power (and showing as separate in tooltip).

I've added this as an option. In the addon's settings you can toggle it on or off (it is ON by default).
You can test it by manually downloading from here: https://github.com/getov/CharacterStatsClassic/releases
I will be uploading it to Curse next week.