Update garnish and render_doc functions

Now all element node attributes have to be threaded through the
eph.attributes which is done by the garnish function.

lib/outlook/internal_tree.ex

@@ -33,6 +33,11 @@ defmodule Outlook.InternalTree do
       options,
       fn prop, opts -> Map.put_new(opts, prop, []) end
       )
+    options = Enum.reduce(
+      ~w(elements tunits)a,
+      options,
+      fn prop, opts -> Map.put_new(opts, prop, %{}) end
+      )
     InternalTree.garnish(tree, options)
   end