Leaking vars into the user's zsh env is not good, esp. when the var
refs a directory -- zsh may then interpret it as a named directory
ref and use it in prompts, for e.g.
Now uses preexec_functions to execute autojump's preexec.
This prevents clobbering of preexec in case its already set.
Signed-off-by: Joel Schaerer <joelthelion@laposte.net>
