Add Zombie "infection" debuff

[douglasg14b]

This would be a temporary debuff applied to units a zombie does armor damage too.

This will need to be fleshed out with more detail than other buffs...

• If a unit takes, say 10% of their health in damage the debuff is applied
  • How to determine this is still unknown
• The debuff works the same as dynamic zombie decay, so it has very little effect on low-health units
• Certain units like commanders should be immune to this effect and support commanders should be resistant
• Only applied to damage done from zombified units
• Some units like static defenses/arty should not apply this effect
• Air -> ground should require more damage to apply this effect?