smart_join_lines.el

;; Join next line or join all lines in region
(defun smart-join-line ()
  "Join the current line with the line beneath it or all region lines."
  (interactive)
  (if (use-region-p)
      (save-excursion
	(let ((start-line (line-number-at-pos (region-beginning)))
	      (current-line (line-number-at-pos (region-end))))
	  (goto-char (region-end))
	  (while (> current-line start-line)
	    (join-line)
	    (setq current-line (line-number-at-pos)))))
    (delete-indentation 1)))

(global-set-key (kbd "C-S-j") 'smart-join-line)