[SPEC] Periscope Rifle

This merits a new Item Extension System.
Various light to medium sized weapons could attach this in order to mod their weapon’s projectile exit.
Oh, great idea! Some ideas of attachments that came to my mind: e.g makes the gun perfectly accurate, make it kick up after each shot or make the bullets bounce from walls. The editor should have an option to spawn items that are already upgraded with chosen extensions.
Basically, a trench rifle modification
I was thinking more of a submarine periscope design

Re-opening to track the port to the latest version of the game.
@zicklag This was implemented in #953 / unless I missed something here :)
Whoops! Awesome. :)