Add tasks for all the new languages

Task/Flatten-a-list/Elm/flatten-a-list.elm

@@ -0,0 +1,27 @@
+import Graphics.Element exposing (show)
+
+type Tree a
+  = Leaf a
+  | Node (List (Tree a))
+
+flatten : Tree a -> List a
+flatten tree =
+  case tree of
+    Leaf a -> [a]
+    Node list -> List.concatMap flatten list
+
+-- [[1], 2, [[3, 4], 5], [[[]]], [[[6]]], 7, 8, []]
+tree : Tree Int
+tree = Node
+  [ Node [Leaf 1]
+  , Leaf 2
+  , Node [Node [Leaf 3, Leaf 4], Leaf 5]
+  , Node [Node [Node []]]
+  , Node [Node [Node [Leaf 6]]]
+  , Leaf 7
+  , Leaf 8
+  , Node []
+  ]
+
+main =
+  show (flatten tree)