Add CVar to enable original Doom health mechanics

* Disables health token system when enabled
* Added logic to set health tokens as a function of player health
* Added CVar-aware spawners for Doom health and armour pickups
* Changed player damage resistance to 0.5 to compensate for OHGODTHEPAIN
* Improves balance in mods designed around the original health system
Merge pull request #18 from ShiftyAxel/doomhealth

Add CVar to enable original Doom health mechanics