Making sorting optional

At present, `helm-org-recent-headings-source` is defined as follows in `helm-org-recent-headings.el`:

```emacs-lisp
(defvar helm-org-recent-headings-source
(helm-build-sync-source " Recent Org headings"
:candidates (lambda ()
(org-recent-headings--prepare-list)
org-recent-headings-list)
:candidate-number-limit 'org-recent-headings-candidate-number-limit
:candidate-transformer 'helm-org-recent-headings--truncate-candidates
;; FIXME: If `helm-org-recent-headings-map' is changed after this `defvar' is
;; evaluated, the keymap used in the source is not changed, which is very confusing
;; for users (including myself). Maybe we should build the source at runtime.
:keymap helm-org-recent-headings-map
:action (helm-make-actions
"Show entry (default function)" 'org-recent-headings--show-entry-default
"Show entry in real buffer" 'org-recent-headings--show-entry-direct
"Show entry in indirect buffer" 'org-recent-headings--show-entry-indirect
"Remove entry" 'helm-org-recent-headings-remove-entries
"Bookmark heading" 'org-recent-headings--bookmark-entry))
"Helm source for `org-recent-headings'.")
```

`org-recent-headings--prepare-list` sorts the history by frecency. I personally don't like this behavior, and I'd prefer history ordered by recency, without duplicates.

I also want to add some custom actions to this Helm source.

For now, I will copy this source code and define my own Helm source for recent headings to suit my own needs. It might be better to allow the user to define his/her own sorting mechanism, e.g. using `prescient`. In the end, I may end up with your implementation, but I don't know if frecency is a good idea here.