added behind direction alias

MomentaryMeeting/Game/CommandParser.cs

@@ -42,6 +42,7 @@ namespace MomentaryMeeting.Game
                 { "backwards", UserModel.RelativeDirection.Back },
                 { "backward", UserModel.RelativeDirection.Back },
                 { "back", UserModel.RelativeDirection.Back },
+                { "behind", UserModel.RelativeDirection.Back },
                 { "left", UserModel.RelativeDirection.Left },
                 { "right", UserModel.RelativeDirection.Right }
             };