Add jump marks to psearch navigation

Inspired by plugins like EasyMotion this adds jump marks to the psearch result buffer for quick access to all displayed results

- checks for CTRL-L in main loop
- toggles a pending-jump flag for next input
- on next input check if entered character is a jump mark and discard it otherwise
- highlights jump marks ('\ <[a-z]>\ ')

I also updated the documentation with a short explanation.