Add On-hit damage mechanism

[PaulQbFeng]

• Have a look at several on hit items, spells ?

[Lysxia]

The BaseSpell class now has a on_hit_effect that you can override, which gets called on every hit_damage call.

https://github.com/PaulQbFeng/too-tanky/blob/cb13bded1838f36fe4796cfd220cee230475c762/tootanky/spell.py#L112

[PaulQbFeng]

First implem done with https://github.com/PaulQbFeng/too-tanky/pull/54