Add some functionality to dired.
Add some functionality to dired.

Include:
- Load current directory with nautilus in dired.
- List directories first.
- Get total size of marked files with du command.

zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d