Add entities, and allow walking around

Add support for entities via a port of the EntityMap type, and implement
command support starting at basic hjkl.

src/Xanthous/Command.hs

@@ -0,0 +1,20 @@
+module Xanthous.Command where
+
+import Graphics.Vty.Input (Key(..), Modifier(..))
+
+import Xanthous.Prelude hiding (Left, Right, Down)
+import Xanthous.Data (Direction(..))
+
+data Command
+  = Quit
+  | Move Direction
+  | PickUp
+  | PreviousMessage
+
+commandFromKey :: Key -> [Modifier] -> Maybe Command
+commandFromKey (KChar 'q') [] = Just Quit
+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 _ _ = Nothing