Add an option to disable friendly fire between enemies

[paulfioravanti]

Although having enemies fight and kill each other can be convenient/funny in standard Doom play, given that Typist is all about typing practice, whenever an enemy kills another enemy, that's one less word I get to practice typing.

It would be nice to have an option of either:

• enemies only attack the player, and never each other, or
• don't allow the enemies to damage each other when they attack; only the player can damage them