Stopped autojump.zsh from clobbering preexec.

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>

autojump.zsh

@@ -1,7 +1,10 @@
-function preexec() {
+function autojump_preexec() {
     (autojump -a "$(pwd -P)"&)>/dev/null
 }
 
+typeset -ga preexec_functions
+preexec_functions+=autojump_preexec
+
 alias jumpstat="autojump --stat"
 
 function j { new_path="$(autojump $@)";if [ -n "$new_path" ]; then echo -e "\\033[31m${new_path}\\033[0m"; cd "$new_path";fi }