====== cd Options ======

''autocd''
  * if a directoryname is entered like a command, and there is no command of that name; the 'cd' command is executed for that directory

''auto_pushd''
  * make cd push the old directory to the dirstack

''cdable_vars''
  * if cd would fail, because the arg is not a dir, try to expand the argument as if it was called the ~expression way

''pushd_ignore_dups''
  * don't push dups on the dirstack

''pushd_to_home''
  * make 'pushd' with no argument, act like ''pushd ${HOME}''