Here is a patch against latest git Org that tries to simplify the
handling of HTML preamble/postamble: I find the interaction between
:preamble and :auto-preamble a bit confusing and a recent questions by
Jeff on customizing org-export-html-preamble raised this issue.

So here is what I suggest:

- `org-export-html-preamble' is a custom variable to tell whether a HTML
  preamble should be included or not;

- `org-export-html-preamble-format' is a format string controlling what
  is included in the preamble;

- In publishing project, everything is done from :html-preamble:

  + when `t', inserts a preamble with the default formatting
  + when an alist, insert a preamble with this alist formatting
  + when nil, no preamble is inserted.

  (Same for postamble).

This also let people use their own HTML classes, language-based
preamble/postamble, etc.

Would people welcome such a simplification?

Thanks!