Merge branch 'master' of github.com:rupa/z

version 1.8 PROMPT_COMMAND, cleanup

version 1.7 x and c options, prompt cmd append, revert exec warning

version 1.6 subdirs, option combining

version 1. exclude dirs

version 1.4 zsh changes (breaks backward compatibility)

version 1.3 tab completion improvements

version 1.2