Add support for 1:many KeyValueTags

[Lothsahn]

Add support for 1:many KeyValueTags in Facepunch Items.

Note: This breaks backwards compatibility for existing clients that might be using this functionality. They'll have to retrieve the first item from the list instead. However, I think this is acceptable because:

1. The current implementation has undefined behavior if there are multiple values for the same key. One of them is returned--which one is based on the order provided by Steamworks, which is not specified in the API
2. This makes Facepunch match the Steamworks implementation
3. Given this code is not currently in any released binaries--it's only available for people who have manually built from source--the backwards compatibility impact seems low.