Alternate approach to appending to PROMPT_COMMAND

Seems to work well no matter what weirdness is already in
PROMPT_COMMAND.

Thanks @godlygeek. Closes #99

z.sh

@@ -230,7 +230,7 @@ elif complete &> /dev/null; then
     [ "$_Z_NO_PROMPT_COMMAND" ] || {
         # bash populate directory list. avoid clobbering other PROMPT_COMMANDs.
         grep -q "_z --add" <<< "$PROMPT_COMMAND" || {
-            PROMPT_COMMAND="${PROMPT_COMMAND%;}${PROMPT_COMMAND:+;}"'_z --add "$(pwd '$_Z_RESOLVE_SYMLINKS' 2>/dev/null)" 2>/dev/null;'
+            PROMPT_COMMAND="$PROMPT_COMMAND"$'\n''_z --add "$(pwd '$_Z_RESOLVE_SYMLINKS' 2>/dev/null)" 2>/dev/null;'
         }
     }
 fi