worf.el (worf-keyword): add and bind to "K"

* worf.el (worf-down): Move by keyword if it's set.
  (worf-up): Move by keyword if it's set.
  (worf--prev-keyword): New function.
  (worf--next-keyword): New function.
  (worf--current-keyword): New variable.
  (worf--invalidate-keyword): New function.