Gunmod support for crossbows!

Adds a new mod category, crossbow.  I think this is something people have wanted for a while, and as it turns out it wasn't nearly as hard as i thought it would be.  Currently, only the laser sight is flagged as being compatible with crossbows, I used it for testing.  I could use some guidance as to what gunmods should be flagged as crossbow-compatible, so please tell me what you think I should add!

Tested and works,  I'll enable more gunmods for crossbows and see if anything breaks.