Added Natural Spawns Only to Undead Army Config

[Peashooter101]

Added option "natural_spawns_only" to the undead army configuration which determines if the undead has to spawn naturally to affect undead army progress.

Defaults to false since before the feature was added, it was inherently "false".

• Adding this option to the mod will inherently not change the original functionality, the config has to be deliberately changed.
• Attempted to follow coding style used in this project.

[Peashooter101]

The intention of this PR...

• Provides server owners and players the option to disregard unnatural spawns (Spawner, Spawn Egg, Summon Command).
• Maintain original functionality and intention while providing a new option.

Tested on Singleplayer and Forge localhost Server (using runClient and runServer).

• With natural_spawns_only = false (default setting)
  • Spawner, Spawn Egg, Natural Spawn will all contribute to undead army progress.
• With natural_spawns_only = true
  • Spawner, Spawn Egg, Summon Command do not contribute to undead army progress.
  • Natural spawn contributes to undead army progress.

Testing was based on feedback from command /undeadarmy progress executed from operator level player.

[Majrusz]

Great job and thanks for the contribution! :D