New variable shell-command-guess-dired-optional (bug#18132)

New variable shell-command-guess-dired-optional (bug#18132)

* doc/emacs/dired.texi (Shell Command Guessing):
Add dired-guess-shell-alist-optional.

* lisp/dired.el (dired-guess-shell-alist-user):
* lisp/dired-aux.el (dired-do-shell-command):
Add dired-guess-shell-alist-optional to docstring.
(dired-guess-shell-alist-default): Move media commands to
'dired-guess-shell-alist-optional'.
(dired-guess-shell-alist-optional): New variable.
(dired-guess-default): Add 'dired-guess-shell-alist-optional'
to the end after 'dired-guess-shell-alist-user'
and 'dired-guess-shell-alist-default'.
(shell-command-guess-functions): Add more options.
(shell-command-guess-dired): Remove function.
(shell-command-guess-dired-user)
(shell-command-guess-dired-default)
(shell-command-guess-dired-optional): New functions.