Add "score" command

Can be used by any user to view the current score of any other user (even across games).  Also allows admins to modify a user's current score (up or down), which can be useful on private servers where admins are playing and can "repair" a score if someone gets disconnected or kicked accidentally.

Could possibly be extended in future to allow game hosts to modify scores of players in the same game, but this would require some thought to abuse prevention.  At least some kind of gamewide announcement.