feat: add damage mitigation to armour

feat: add damage mitigation to armour

Damage mitigation (up to 99%) is now available on every piece of armour.

When you're hit, if that piece of armour has damage mitigation you'll
take that percentage of damage less.

IE: Damage Mitigation = 10% then you'll take 90% of the damage

This should since it feels like armour doesn't really do too much.

Fixes: #3