Adding Modifier objects.

Modifier objects contain data about an existing temporary modification. Each is given a unique ID to check for stacking.

Nothing but a basic implementation has been done so far, but EmpowerOther demonstrates how to prevent stacking using a Modifier.

https://app.asana.com/0/18992600168310/19488182624757
Merge branch 'refactor/buffs-debuffs' of https://github.com/Covalent-Games/pewpewspace into refactor/buffs-debuffs