Add a previous message command

Add a "previous message" command, triggered via ctrl+p.

I attempted here to get the message area to still take up a row of space
post-hiding the message, but failed - should revisit that at some point

src/Xanthous/Command.hs

@@ -8,8 +8,8 @@ import Xanthous.Data (Direction(..))
 data Command
   = Quit
   | Move Direction
-  | PickUp
   | PreviousMessage
+  -- | PickUp
 
 commandFromKey :: Key -> [Modifier] -> Maybe Command
 commandFromKey (KChar 'q') [] = Just Quit
@@ -17,4 +17,7 @@ commandFromKey (KChar 'h') [] = Just $ Move Left
 commandFromKey (KChar 'j') [] = Just $ Move Down
 commandFromKey (KChar 'k') [] = Just $ Move Up
 commandFromKey (KChar 'l') [] = Just $ Move Right
+
+commandFromKey (KChar 'p') [MCtrl] = Just PreviousMessage
+
 commandFromKey _ _ = Nothing