Parameterize some html content containers
* lisp/ox-html.el: (define-backend): Add :html-doctype and
:html-container parameters.
(org-html-doctype): New option for doctype declaration.
(org-html-container-elemnt): New option for specifying the wrapper
container element.
(org-html-divs): Change to alist of three entries each containing a
key ('preamble, 'content, 'postamble), an HTML element type and an
id to allow setting container elements.
(org-html--build-preamble): Modified to use `org-html-divs'.
(org-html--build-postamble): Modified to use `org-html-divs'.
(org-html-template): Modified to use doctype and container-element
settings.