Plague 3.18

Not a huge amount done this week, I've been busy with other stuff. This release has tidies up some loose ends from last weeks release.

• improved “fire_escape” ai algorithm
  
• added Sean Lange's burnt shrubs art
  
• zombie_fast now has a burnt corpse, thanks to Sean Lange
  
• allowed the player to pick the formation (use key “c”)
  
• added wedge, line, None formations (previously only ring formation)
  
• changed shotgun number to 3 (better than SMG, worse than AR)