Add an option to disable killing multiple enemies with a single shot, regardless of weapon

[paulfioravanti]

Although being able to kill multiple enemies with a single shot depending on the weapon in standard Doom play is great, given that Typist is all about typing practice, being able to kill multiple enemies at once means I practice typing less words, and unconsciously try and use easier words to kill more enemies at once.

It would be nice to be able to have an option that can switch off multi-kills.