Allow having dividers between columns in vtable

Allow having dividers between columns in vtable

* doc/misc/vtable.texi (Making A Table): Document it.
* lisp/emacs-lisp/vtable.el (vtable): Add a divider slot.
(make-vtable): Accept :divider and :divider-width arguments.
(vtable--insert-line, vtable--insert-header-line): Display the
divider.