Rewrite the handling of HTML preamble/postamble.
* org-html.el (org-export-html-auto-preamble)
(org-export-html-auto-postamble): Remove.
(org-export-html-preamble, org-export-html-postamble): Turn
into custom variables. Update the docstrings.
(org-export-html-preamble-format)
(org-export-html-postamble-format): New custom variables.
(org-export-as-html): Use org-export-html-postamble-format and
org-export-html-preamble-format.
(org-export-html-title-format): delete.
* org-exp.el (org-export-plist-vars): Remove
:auto-preamble and :auto-postamble. Rename :preamble and
:postamble to :html-preamble and :html-postamble.
* org-publish.el (org-publish-project-alist): Remove
:auto-preamble and :auto-postamble. Rename :preamble and
:postamble to :html-preamble and :html-postamble.
* org.texi (Publishing options): replace :preamble and
:auto-preamble by :html-preamble (same for postamble.)