Implement Endurance Modifications

Forked from #10 (closed)

Endurance goes down by 1 pt per combat round. At 0 it counts down CON. For each point below zero rolls get -1. At -CON character falls unconscious.

This needs to be implemented in combat counter. CSS needs to show current Endurance and max, and we need a button to reset after combat.

We'll implement this late in the project, if at all.