Refactor yon-format-post-number function.

* yon-navigation.el (yon-current-buffer-board)
(yon-browse-thread-other-window): New functions.

* yon-chan-tests.el (test-yon-current-buffer-thread)
(test-yon-browse-thread-other-window): New tests.

* yon-formatting.el (yon-format-post-number): Refactor.
Add 0.1 release TODO items.

* TODO (0.1 release): New items.