Use fish function -on-event and -on-variable

fish_prompt doesn't run any test anymore and merely prints $LP_PROMPT
- _lp_prompt is called on 'fish_prompt' events (fish fires this event
  whenever a new prompt is about to be displayed).
- _lp_directory is called only when $PWD changes value of on
  'lp_dir_option_changed' events.
  * There is now no need to keep an $OLD_PWD variable and compare it
    with the value of $PWD at each call of fish_prompt
  * The event 'lp_dir_option_changed' is fired when 'lp_enable' or
    'lp_disable' change an option involved in '_lp_directory'.
- _lp_checks is called only on 'lp_feature_option_changed' events.
  The event 'lp_feature_option_changed' is fired when 'lp_enable' and
  'lp_disable' change an option involved in '_lp_checks'.
Use fish_title to set the term title
Complete cvs functions