Support string attributes

[Powerlord]

I haven't checked if there are anything useful in string attributes, but for completeness sake, it'd be nice if TF2Items supported the new string attributes.

[asherkin]

I haven't checked if there are anything useful in string attributes, but for completeness sake, it'd be nice if TF2Items supported the new string attributes.

Based on FlaminSarge's investigation, the strings aren't networked between the client and server.