all-done.el

(defun jb/slack-quote-region ()
    (with-temp-buffer
      (insert region)
      (goto-char 1)
      (while (> (point-max) (point))
        (beginning-of-line)
        (insert "> ")
        (forward-line 1))
      (buffer-string)))

(defun jb/decorate-text (text)
  (let* ((decorators '(("None" . (lambda (text) text))
                       ("Code"  . (lambda (text) (concat "```" text "```")))
                       ("Quote"  . (lambda (text) (jb/slack-quote-region text)))))
         (decoration (completing-read "Select decoration: "
                                      decorators
                                      nil
                                      t)))
    (funcall (cdr (assoc decoration decorators)) text)))

(defun jb/send-region-to-slack ()
  (interactive)
  (let* ((team (slack-team-select))
         (room (slack-room-select
                (cl-loop for team in (list team)
                         append (with-slots (groups ims channels) team
                                  (append ims groups channels))))))
    (slack-message-send-internal (jb/decorate-text (filter-buffer-substring
                                                    (region-beginning) (region-end)))
                                 (oref room id)
                                 team)))