Allow filtering a package by MUDs

[vadi2]

I reckon most packages right now are MUD-specific - so in a central repository, having packages show up by default that have no relevance to you would make the listing meaningless. It should be possible for a package author to specify what game(s), or all, should this package apply to.

One tricky point is how to do this - game URL seems like a good idea, but then if someone changes their profile to connect to the IP instead (rare, but valid thing to do), the filtering would break. The package manager could either: allow packages to include game IPs to filter on as well (but this is bad as IPs can change), or the package manager should keep a reverse IP -> game URL itself database that's updated often enough to solve this.