org-element: Add a new greater element (section)

org-element: Add a new greater element (section)

* contrib/lisp/org-element.el (org-element-section-parser,
  org-element-section-interpreter): New functions
(org-element-greater-elements): Add new element to the list.
(org-element-at-point): Change arguments to handle a new section mode.
(org-element-guess-type): Accept an optional argument to look for
  sections in priority.
(org-element-parse-buffer): Start in section mode by default.  Thus
  any text before the first headline is still in a section of his
  own.
(org-element-parse-elements): Handle new section mode.