Skill parameters do not support placeholders

Summary

Describe the bug encountered in detail.

Steps to reproduce

Provide a full set of steps used to reproduce the bug, including links to SEPARATE Pastebin pastes for each relevant configuration file.

Current behavior

If you use skill parameters, when you run the metaskill from an item/mob, you can't use placeholders for the parameters

Let's use an example:

Burn:
  Skills:
  - damage{a=<skill.damage>;type=FIRE}
  - ignite{duration=<skill.duration>*20}

Mob:
  Type: ZOMBIE
  Damage: 5
  Skills:
  - skill{s=Burn;damage=<caster.damage>*2;duration=<caster.damage>} ~onAttack

Intended correct behavior

What should happen is the skill will damage for 10 damage, and ignite for 5 seconds. However, instead, it errors out with a placeholder error.

Proposed fixes

Let it support placeholders/math/etc.