fix a bug / modify keybinds for backward-search

- bug reported in https://github.com/zk-phi/phi-search/issues/15 is now
fixed (sorry for inconvenience)
- use phi-search-additional-keybinds also in backward-search
update doc
accept prefix-argument

https://github.com/zk-phi/phi-search/pull/14
tiny fix for the last commit