Preview buffer as though current item were inserted

This is implemented using an overlay which is created when
browse-kill-ring-setup is called and deleted on
browse-kill-ring-quit. Because this setup might not work for everyone,
you can disable it using the customize variable
browse-kill-ring-show-preview.

- browse-kill-ring-preview-update: new function, called as a
  post-command-hook, that updates the overlay to show the contents of
  the current item.

- browse-kill-ring-current-string now takes an optional `no-error'
  parameter. If t, this function returns nil instead of erroring.