Resolved #2: forbid leaving a room

You can use `@exit()` to set `canExit` and print the "YOU SHALL NOT LEAVE"
message.

lib/room.coffee

@@ -24,6 +24,7 @@ class SaletRoom
     @priority = 1
     @displayOrder = 1
     @canSave = true
+    @canExit = true
     @tags = []
     @choices = ""
     @optionText = "Choice"
@@ -63,6 +64,14 @@ class SaletRoom
       Also if f == this.name (we're in the same location) the `before` and `after` callbacks are ignored.
     ###
     @entering = (system, f) =>
+      if (
+        f != @name and
+        f? and
+        system.rooms[f].canExit? and
+        system.rooms[f].canExit == false
+      )
+        return system.goTo(f, f)
+
       if @clear and f?
         system.view.clearContent()
       else