MachineGun

• Adds a rebindable key (default 'f') to use the Shotgun in Machine Gun mode. Hold it down to continuously fire at a configurable rate (in shots per second). (NEW) Also knocks back the player on every shot with a configurable force.

• Currently gives the Shotgun infinite ammo (may have to load it initially). However, in the future I'd like to see Ammo become Magazines and for the Shotgun to get a new skin.