Support exporting Org Special Blocks like ox-html

It works almost the same way as ox-html, but a little hack is add to
get around a Hugo/Blackfriday limitation that Markdown text cannot be
wrapped in HTML elements like div. Details:
https://github.com/kaushalmodi/ox-hugo/issues/93.

Fixes https://github.com/kaushalmodi/ox-hugo/issues/105.

ox-blackfriday.el

@@ -83,6 +83,7 @@ Note that this variable is *only* for internal use.")
                      (plain-list . org-blackfriday-plain-list)
                      (plain-text . org-blackfriday-plain-text)
                      (quote-block . org-blackfriday-quote-block)
+                     (special-block . org-blackfriday-special-block)
                      (src-block . org-blackfriday-src-block)
                      (strike-through . org-blackfriday-strike-through)
                      (table-cell . org-blackfriday-table-cell)
@@ -524,6 +525,41 @@ communication channel."
             (when next-is-quote
               "\n\n<!--quoteend-->"))))
 
+;;;; Special Block
+(defun org-blackfriday-special-block (special-block contents _info)
+  "Transcode a SPECIAL-BLOCK element from Org to HTML.
+CONTENTS holds the contents of the block.
+
+This function is adapted from `org-html-special-block'."
+  (let* ((block-type (org-element-property :type special-block))
+         (html5-fancy (member block-type org-html-html5-elements))
+         (attributes (org-export-read-attribute :attr_html special-block)))
+    (unless html5-fancy
+      (let ((class (plist-get attributes :class)))
+        (setq attributes (plist-put attributes :class
+                                    (if class
+                                        (concat class " " block-type)
+                                      block-type)))))
+    (let* ((contents (or contents ""))
+           ;; If #+NAME is specified, use that for the HTML element
+           ;; "id" attribute.
+	   (name (org-element-property :name special-block))
+	   (attr-str (org-html--make-attribute-string
+	              (if (or (not name) (plist-member attributes :id))
+		          attributes
+		        (plist-put attributes :id name))))
+	   (attr-str (if (org-string-nw-p attr-str)
+                         (concat " " attr-str)
+                       "")))
+      ;; The empty HTML element tags like "<div></div>" is a hack to
+      ;; get around a Blackfriday limitation.  Details:
+      ;; https://github.com/kaushalmodi/ox-hugo/issues/93.
+      (if html5-fancy
+	  (format "<%s%s>\n<%s></%s>\n\n%s\n</%s>"
+                  block-type attr-str block-type block-type contents block-type)
+	(format "<div%s>\n<div></div>\n\n%s\n</div>"
+                attr-str contents)))))
+
 ;;;; Src Block
 (defun org-blackfriday-src-block (src-block _contents info)
   "Transcode SRC-BLOCK element into Blackfriday Markdown format.

test/site/content-org/all-posts.org

@@ -2645,6 +2645,52 @@ Line 3 had no =>= char.
 > ← See that this =>= on line 4 is retained even at the beginning of the line.
 Line 5 has this > charcter in-between and is retained.
 #+END_VERSE
+* Special Blocks                                              :special_block:
+:PROPERTIES:
+:EXPORT_FILE_NAME: special-blocks
+:END:
+** HTML5 Element Blocks
+*** Block without NAME, class or id
+#+BEGIN_article
+This is /an article/.
+#+END_article
+*** Block with NAME
+#+NAME: Aside A
+#+BEGIN_aside
+/Some/ *text* --- 1
+
+| a | b | c |
+| d | e | f |
+#+END_aside
+*** Block with class and id
+#+ATTR_HTML: :class my-section :id section-a
+#+BEGIN_section
+/Some/ *text* --- 2
+
+| g | h | i |
+| j | k | l |
+#+END_section
+** DIV-wrapped Blocks
+*** DIV without NAME, class or id
+#+BEGIN_something
+This is /some text/ wrapped in a =div= block with class =something=.
+#+END_something
+*** DIV with NAME
+#+NAME: Foo A
+#+BEGIN_foo
+/Some/ *text* --- 3
+
+| m | n | o |
+| p | q | r |
+#+END_foo
+*** DIV with class and id
+#+ATTR_HTML: :class my-bar :id bar-a
+#+BEGIN_bar
+/Some/ *text* --- 4
+
+| s | t | u |
+| v | w | x |
+#+END_bar
 * Org TODO keywords                                                    :todo:
 ** Post with a TODO heading
 :PROPERTIES:

test/site/content/posts/special-blocks.md

@@ -0,0 +1,86 @@
++++
+title = "Special Blocks"
+tags = ["special-block"]
+draft = false
++++
+
+## HTML5 Element Blocks {#html5-element-blocks}
+
+
+### Block without NAME, class or id {#block-without-name-class-or-id}
+
+<article>
+<article></article>
+
+This is _an article_.
+
+</article>
+
+
+### Block with NAME {#block-with-name}
+
+<aside id="Aside A">
+<aside></aside>
+
+_Some_ **text** --- 1
+
+| a | b | c |
+|---|---|---|
+| d | e | f |
+
+</aside>
+
+
+### Block with class and id {#block-with-class-and-id}
+
+<section class="my-section" id="section-a">
+<section></section>
+
+_Some_ **text** --- 2
+
+| g | h | i |
+|---|---|---|
+| j | k | l |
+
+</section>
+
+
+## DIV-wrapped Blocks {#div-wrapped-blocks}
+
+
+### DIV without NAME, class or id {#div-without-name-class-or-id}
+
+<div class="something">
+<div></div>
+
+This is _some text_ wrapped in a `div` block with class `something`.
+
+</div>
+
+
+### DIV with NAME {#div-with-name}
+
+<div class="foo" id="Foo A">
+<div></div>
+
+_Some_ **text** --- 3
+
+| m | n | o |
+|---|---|---|
+| p | q | r |
+
+</div>
+
+
+### DIV with class and id {#div-with-class-and-id}
+
+<div class="my-bar bar" id="bar-a">
+<div></div>
+
+_Some_ **text** --- 4
+
+| s | t | u |
+|---|---|---|
+| v | w | x |
+
+</div>