fix typo/interesting bug

there are both `posts` and `model.posts`, but because we used `posts` (a
constant) in place of `model.posts` in the update function, we could
only every post one new post.

elm/Stream.elm

      AddPost _ -> { model |
                      isEditing <- False,
                      posts <- let p = model.newPost
                               in { p | created <- model.referenceDate } :: posts }
                               in { p | created <- model.referenceDate } :: model.posts }
      CancelPost -> { model | isEditing <- False, newPost <- Post.empty }