* slime.el (slime-repl-banner-function): New hook.
(slime-repl-update-banner): Use it and reset markers after calling
it.
(slime-set-default-directory): Don't call slime-repl-update-banner
here.
(slime-repl-insert-prompt): Set slime-repl-input-end-mark to
point-max.

* contrib/slime-banner.el: New file