Add ELTS to LIST-VAR and save for future sessions, safely.
ELTS should be a list. This function adds each entry to the
value of LIST-VAR using `add-to-list'.
If Emacs is initialized, call `customize-save-variable' to save
the resulting list value now. Otherwise, add an entry to
`after-init-hook' to save it after initialization. (fn LIST-VAR ELTS)