Add permission to receive and config toggle for how players are counted

Added the permission node <code>randomgift.receive</code> that should be givin to all players who are eligible to receive gifts.  This makes it possible to only give certain players/groups this added bonus.

Added configuration option <code>all-players</code> to toggle if all players should be counted towards minimum online player count, or only count players with <code>randomgift.receive</code> permission node.