Clean handling :html-pre/postamble options wrt author/email/creator-info.
* org-html.el (org-export-html-preamble)
(org-export-html-postamble): now default to `nil'.
(org-export-as-html): when :html-pre/postamble is nil, fall
back on the default pre/postamble, which depends on the
:author-info, :email-info, :creator-info options.
* org-exp.el (org-export-plist-vars): reorder the alist.
* org.texi (Export options): better document :html-preamble
and :html-postamble: setting these options will override any
:author-info, :email-info and :creator-info options for the
HTML export.