New build: menu option for configuring pistol start behavior

This build adds a new section to the GZDoom options menu that offers a single option: control over the mod's "pistol start" behavior.  The previous behavior is now considered the default, ie that you pistol start every level of every episode except Cabal, while options for "always pistol start" and "never pistol start" have been added. Under the hood this uses the same logic as my Always Pistol Start mod.