Add Ctrl+p, jump to previous unread channel

* Introduce function `selectAdjacentUnreadChannel`, which takes a
  function argument to determine the channel to pick, from an ordered
  list of channels with activity.
* Use it in `nextActive`.
* Introduce `prevActive`, which jumps to the closest buffer with unread
  activity as if navigating via Ctrl+Alt+Up.
* Add a keybinding and document it in the README.