Fix handling of internal links and new html doctype option.

Fix handling of internal links and new html doctype option.

* contrib/lisp/ox-deck.el (deck): Add deck specific link function.
(org-deck-toc): Generate correct link to slide for custom ids as well
as default section ids.
(org-deck-link): HTML custom ids link to the headline. deck.js
requires linking to the wrapper (slide) so modify the link to point at
the container instead of the headline.
(org-deck-template): Fix doctype handling to use alist option instead
of literal string.