Allow mixed export of numbered and unnumbered sections in HTML
* lisp/org-html.el (org-export-as-html): Get local value of
org-export-with-section-numbers from the buffer's plist. Deal
specially with the case the resulting value is an integer.
(org-html-level-start): New optional argument of the option plist used
instead of `org-export-with-section-numbers'. Also deal specially
with the case that the value is an integer.
When `org-export-with-section-numbers' (or the buffer-local
:section-numbers option) is an integer, we now export the first NUM
levels of headings with numbers and lower-level headings without.