Use zshaddhistory to log command in zsh

Previously multi-line command was badly escaped (containing "\\\n").
Using zshaddhistory hook solves this problem.
Fix how _RASH_COMMAND is reset in zsh

As preexec is called after zshaddhistory, _RASH_COMMAND must be reset
in _rash-precmd just before exiting.
Add _rash-history-fallback
Merge branch 'zshaddhistory'

Fixed a bug with multi-line command in zsh.