Bash support for Zsh like chpwd hook

Implemented based on the description from http://zsh.sourceforge.net/Doc/Release/Functions.html#Hook-Functions

Usage

1. load function.sh and load.sh, eg:

   Copy

    source chpwd/functions.sh
    source chpwd/load.sh
   

2. add the hook - replace _hook_name with your function name:

   Copy

    export -a chpwd_functions                              # define hooks as an shell array
    [[ " ${chpwd_functions[*]} " == *" _hook_name "* ]] || # prevent double addition
    chpwd_functions+=(_hook_name)                          # finally add it to the list