Add M320 as a underslung grenade launcher

This adds a M320 version that can be attached to a rifle, mainly motivated by the way it works in real life. The attachable version can be crafted into the (current) standalone version, and disassembled back into a stock and the attachable version. The weights should be semi-accurate from Googling around. 

I'm putting a [CR] on this since I'm unsure about the numbers. As it stands, the M320 is about as effective as the pipe 40mm launcher. On one hand, it makes sense as the M320 is lighter and more compact, but on the other hand it seems like a factory-made weapon should be better than a home-made one. I guess the thing could also match the M203 in performance instead, but I wouldn't want to make the attached version significantly different from the stand-alone.