Update s5 and deck,js slideshow exporters to work with refactored html

Update s5 and deck,js slideshow exporters to work with refactored html
backend.

* lisp/ox-html.el (org-html--build-pre/postamble): Add css class to
  wrapper div (defaults to `org-pre/postamble-class').  Move spec
  building to separate function (`org-html-format-spec').
(org-html-format-spec): New function.
(org-pre/postamble-class): New variable.

* contrib/lisp/ox-deck.el: Replace :html-{pre,post}amble export
  options with :deck versions.
(org-deck-postamble): Replaces `org-deck-postamble-template', uses
format-spec (and `org-html-format-spec') instead of custom template
option.
(org-deck-preamble): Replaces `org-deck-postamble-template', uses
format-spec (and `org-html-format-spec') instead of custom template
option.
(org-deck-title-slide-template): Use standard format specs.
(org-deck-template): Use org-html--build-pre/postamble instead of
`org-deck-fill-template'. Use fill-template for title slide (with
standard format specs) instead of custom `org-fill-template'
(org-deck-template-alist):  Delete.
(org-deck-postamble-template): Delete.
(org-deck-preamble-template): Delete.

* contrib/lisp/ox-s5.el: Same overall set of changes as ox-deck.
(org-s5-divs): New option, similar usage to `org-html-divs'.
(org-s5-footer-template): Delete.
(org-s5-header-template): Delete.