Add doors and an open command

Add a Door entity and an Open command, which necessitated supporting the
direction prompt. Currently nothing actually puts doors on the map,
which puts a slight damper on actually testing this out.

src/Xanthous/Game/Draw.hs

@@ -41,6 +41,8 @@ drawPromptState (WaitingPrompt msg (Prompt _ pt ps _)) =
   case (pt, ps) of
     (SStringPrompt, StringPromptState edit) ->
       txt msg <+> renderEditor (txt . fold) True edit
+    (SDirectionPrompt, DirectionPromptState) ->
+      txt msg
     _ -> undefined
 
 drawEntities