Fix: (org-ql-select) Properly accept arbitrary sort function

Fixes #37.  Thanks to Milan Zamazal (@mz-pdm).

README.org

@@ -474,6 +474,7 @@ Expands into a call to ~org-ql-select~ with the same arguments.  For convenience
 +  Don't overwrite bindings in =org-agenda-mode-map=.
 +  Don't search buffers without headings, and show a message if the user attempts it.
 +  Don't search hidden/special buffers.
++  Properly accept arbitrary sort functions in =org-ql-select=, etc.  (Fixes [[https://github.com/alphapapa/org-ql/issues/37][#37]].  Thanks to [[https://github.com/mz-pdm][Milan Zamazal]].)
 
 *Compatibility*
 +  Fixes for compatibility with Org 9.2.  (Thanks to [[https://github.com/ataias][Ataias Pereira Reis]] and [[https://github.com/dakra][Daniel Kraus]].)

org-ql.el

@@ -217,15 +217,14 @@ non-nil."
     ;; Sort items
     (pcase sort
       (`nil items)
-      ((guard (and sort
-                   (setq sort (-list sort))
-                   (cl-loop for elem in sort
-                            always (memq elem '(date deadline scheduled todo priority)))))
+      ((or 'date 'deadline 'scheduled 'todo 'priority
+           (guard (cl-loop for elem in sort
+                           always (memq elem '(date deadline scheduled todo priority)))))
        ;; Default sorting functions
-       (org-ql--sort-by items sort))
+       (org-ql--sort-by items (-list sort)))
       ;; Sort by user-given comparator.
       ((pred functionp) (sort items sort))
-      (_ (user-error "SORT must be either nil, or one or a list of the defined sorting methods (see documentation)")))))
+      (_ (user-error "SORT must be either nil, one or a list of the defined sorting methods (see documentation), or a comparison function of two arguments")))))
 
 (cl-defun org-ql-query (&key (select 'element-with-markers) from where narrow order-by)
   "Like `org-ql-select', but arguments are named more like a SQL query.