Export to various flavors of (X)HTML

Export to various flavors of (X)HTML

* lisp/ox-html.el: (org-html-doctype-alist): New variable holding an
alist of (X)HTML doctypes
(org-html-xhtml-p): New function
(org-html-html5-p): New function
(org-html-close-tag): New function
(org-html-html5-fancy): New export option, determining whether or not to
use HTML5-specific elements.
(org-html-html5-elements): New variable, new HTML5 elements.
(org-html-special-block): Export special blocks to new HTML5 elements.
(org-html-format-inline-image): Use <figure> and <figcaption> for
standalone images.
(org-html-format-inline-image,org-html--build-meta-info)
(org-html--build-head,org-html--build-pre/postable,org-html-template)
(org-html-horizontal-rule,org-html-format-list-item)
(org-html-line-break,org-html-table,`org-html-verse-block): Changes to
allow flavored export.

* doc/org.texi: Document export to (X)HTML flavors.