org-agenda: Fix `org-agenda-custom-command' customization

* lisp/org-agenda.el (org-agenda-custom-commands-local-options): Quote
  symbols.

Reported-by: Niall Dooley <dooleyn@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-08/msg00659.html>

lisp/org-agenda.el

@@ -324,12 +324,14 @@ the daily/weekly agenda, see `org-agenda-skip-function'.")
 			 (string))
 		   (list :tag "Number of days in agenda"
 			 (const org-agenda-span)
-			 (choice (const :tag "Day" day)
-				 (const :tag "Week" week)
-				 (const :tag "Fortnight" fortnight)
-				 (const :tag "Month" month)
-				 (const :tag "Year" year)
-				 (integer :tag "Custom")))
+			 (list
+			  (const :format "" quote)
+			  (choice (const :tag "Day" day)
+				  (const :tag "Week" week)
+				  (const :tag "Fortnight" fortnight)
+				  (const :tag "Month" month)
+				  (const :tag "Year" year)
+				  (integer :tag "Custom"))))
 		   (list :tag "Fixed starting date"
 			 (const org-agenda-start-day)
 			 (string :value "2007-11-01"))