Add Completions Auto Update minor mode

Add Completions Auto Update minor mode

This adds a new global minor that updates the *Completions* buffer as
you type in the minibuffer.

* lisp/minibuffer.el (completions-auto-update-idle-time): New option.
(completions-auto-update-timer): New buffer-local variable.
(completions-auto-update)
(completions-auto-update-start-timer)
(completions-auto-update-setup)
(completions-auto-update-exit): New functions.
(completions-auto-update-mode): New global minor mode.
* doc/emacs/mini.texi (Completion Options): Document it.
* etc/NEWS: Announce it.