counsel-yank-pop: add a defcustom, which can set SEPARATOR

[tumashu]

Like the below:

    (defun counsel--yank-pop-format-function (cand-pairs)
      (ivy--format-function-generic
       (lambda (str)
         (mapconcat
          (lambda (s)
            (ivy--add-face s 'ivy-current-match))
          (split-string
           (counsel--yank-pop-truncate str) "\n" t)
          "\n"))
       (lambda (str)
         (counsel--yank-pop-truncate str))
       cand-pairs
       (concat "\n" (make-string 70 ?-) "\n")))

[abo-abo]

Thanks.