Hydration pack (Semi-WIP) (CR)

Make a basic system for hydration packs (commonly known as "CamelBak"s from the brand name). Allows sheathing a 1.5 and a 3 liter hydration bag into either a survivor pack, survivor runner pack, runner pack, or a MOLLE pack. You can then (a)ctivate your pack to drink. Pretty mundane, but saves a bit of volume. I'm not totally sure on the code for the hydration_pack method in iuse.cpp, but I plan to learn to compile for proper testing soon.
As I'm reading this, it'll tell me to "never mind" if I (not paying attention to Git) but haven't got a hydration bag.  Something like "You could install a filled hydration bag into this, if you had one" would be better, as would be adding a line to the packs' descriptions noting that they support hydration-bag installation.