Missing hitgroups mappings

[AdotBdot]

I have encountered an error while parsing hit groups from a demo file and found out that there are missing string mappings for hit group '8' which, after searching in other demo parsing libraries, is assigned for 'neck'.

Solution is easy. Simply add 8_i32 => "neck" to the HIT_GROUP map in maps.rs

I need to stick for workaround for know because I don't use rust and cannot compile the lib by my self so I hope for quick patch.