ox-html: Use OPTIONS items instead of keywords for booleans

* lisp/ox-html.el (html): Replace "HTML_HTML5_FANCY",
  "HTML_INCLUDE_STYLE" and "HTML_INCLUDE_SCRIPTS" with, respectively,
  ":html5-fancy", "html-style" and "html-scripts" options.
(org-html-html5-fancy): Reformat docstring.
* doc/org.texi (HTML doctypes): Reflect keyword removal.
(CSS support): Reflect keyword removal.

The reasoning behind this change is that keywords can only store
strings as value.  Therefore, they are not suited for toggles ("nil"
vs nil problem).

doc/org.texi

@@ -11055,7 +11055,7 @@ See the variable @code{org-html-doctype-alist} for details.  The default is
 
 HTML5 introduces several new element types.  By default, Org will not make
 use of these element types, but you can set @code{org-html-html5-fancy} to
-@code{t} (or use the corresponding @code{HTML_HTML5_FANCY} export option), to
+@code{t} (or set @code{html5-fancy} item in an @code{OPTIONS} line), to
 enable a few new block-level elements.  These are created using arbitrary
 #+BEGIN and #+END blocks. For instance:
 
@@ -11352,9 +11352,9 @@ Each exported file contains a compact default style that defines these
 classes in a basic way@footnote{This style is defined in the constant
 @code{org-html-style-default}, which you should not modify.  To turn
 inclusion of these defaults off, customize
-@code{org-html-head-include-default-style} or set @code{#+HTML_INCLUDE_STYLE}
-to @code{nil} on a per-file basis.}.  You may overwrite these settings, or add to
-them by using the variables @code{org-html-head} and
+@code{org-html-head-include-default-style} or set @code{html-style} to
+@code{nil} in an @code{OPTIONS} line.}.  You may overwrite these settings, or
+add to them by using the variables @code{org-html-head} and
 @code{org-html-head-extra}.  You can override the global values of these
 variables for each file by using these keywords:
 

lisp/ox-html.el

@@ -116,7 +116,7 @@
     (:html-link-org-as-html nil nil org-html-link-org-files-as-html)
     (:html-doctype "HTML_DOCTYPE" nil org-html-doctype)
     (:html-container "HTML_CONTAINER" nil org-html-container-element)
-    (:html-html5-fancy "HTML_HTML5_FANCY" nil org-html-html5-fancy)
+    (:html-html5-fancy nil "html5-fancy" org-html-html5-fancy)
     (:html-link-home "HTML_LINK_HOME" nil org-html-link-home)
     (:html-link-up "HTML_LINK_UP" nil org-html-link-up)
     (:html-mathjax "HTML_MATHJAX" nil "" space)
@@ -124,8 +124,8 @@
     (:html-preamble nil "html-preamble" org-html-preamble)
     (:html-head "HTML_HEAD" nil org-html-head newline)
     (:html-head-extra "HTML_HEAD_EXTRA" nil org-html-head-extra newline)
-    (:html-head-include-default-style "HTML_INCLUDE_STYLE" nil org-html-head-include-default-style newline)
-    (:html-head-include-scripts "HTML_INCLUDE_SCRIPTS" nil org-html-head-include-scripts newline)
+    (:html-head-include-default-style nil "html-style" org-html-head-include-default-style)
+    (:html-head-include-scripts nil "html-scripts" org-html-head-include-scripts)
     (:html-table-attributes nil nil org-html-table-default-attributes)
     (:html-table-row-tags nil nil org-html-table-row-tags)
     (:html-xml-declaration nil nil org-html-xml-declaration)
@@ -919,9 +919,8 @@ publishing, with :html-doctype."
   :type 'string)
 
 (defcustom org-html-html5-fancy nil
-  "When exporting to HTML5, set this to t to use new HTML5
-  elements. This variable is ignored for anything other than
-  HTML5.
+  "Non-nil means using new HTML5 elements.
+This variable is ignored for anything other than HTML5 export.
 
 For compatibility with Internet Explorer, it's probably a good
 idea to download some form of the html5shiv (for instance