empty the post when adding a new post

that might be handled in multiple places, though?

elm/Stream.elm

@@ -65,6 +65,7 @@ update action model =
      AddPost _ -> { model |
                      isEditing <- False,
                      newPost <- Post.empty,
                      posts <- let p = model.newPost
                               in { p | created <- model.referenceDate } :: model.posts }