Add a lobby option to control spawning creeps (viceroids/worms)

The lobby option is only exposed in d2k currently, not in TD or TS.

OpenRA.Game/Network/Session.cs

@@ -180,6 +180,7 @@ namespace OpenRA.Network
 			public bool Dedicated;
 			public string Difficulty;
 			public bool Crates = true;
+			public bool Creeps = true;
 			public bool Shroud = true;
 			public bool Fog = true;
 			public bool AllyBuildRadius = true;