Fixed #254. An option line for specifying external plugins.

Readme.org

@@ -577,20 +577,27 @@
 
 ** Third-Party Plugins
 Reveal.js is also extensible through third-party plugins. Org-reveal
-now includes a mechanism to load these as well. It's a little more
-complicated, because we need to store the specific javascript loading
-code in a defcustom.
+now includes a mechanism to load these as well, either for all org buffers
+by defining variable, or for local org buffer by setting option line.
 
+*** By Defining Variable
 Store the names and loading instructions for each plugin in the
-defcustom ~org-reveal-external-plugins~. This defcustom is an
-associative list. The first element of each Assoc cell is a symbol --
-the name of the plugin -- and the second is a string that will be
-expanded by the ~format~ function when the plugin is loaded. So, this
-second element should have the form ~" {src:
-\"%srelative/path/toplugin/from/reveal/root.js\"}'.  If you need the
+customizable variable ~org-reveal-external-plugins~. This variable is
+an associative list. The first element of each Assoc cell is a symbol
+-- the name of the plugin -- and the second is the string of the code
+for enabling the plugin. The string can have ONE optional ~%s~, which
+will be replaced by `reveal-root`.
+
+So, this second element should have the form ~"{src:
+\"%srelative/path/toplugin/from/reveal/root.js\"}~.  If you need the
 async or callback parameters, include those too.  Ox-reveal will add
 the plugin to the dependencies parameter when Reveal is initialized.
 
+*** By Local Option Line
+Specify the plugin by option line ~#+REVEAL_EXTERNAL_PLUGINS: string
+of code~. Similiar to the global plugin definition, the string of code
+can have one optional ~%s~ to be replaced by ~reveal-root~.
+
 ** Highlight Source Code
 
    There are two ways to highlight source code.

ox-reveal.el

@@ -90,6 +90,7 @@
     (:reveal-slide-header "REVEAL_SLIDE_HEADER" nil org-reveal-slide-header t)
     (:reveal-slide-footer "REVEAL_SLIDE_FOOTER" nil org-reveal-slide-footer t)
     (:reveal-plugins "REVEAL_PLUGINS" nil nil t)
+    (:reveal-external-plugins "REVEAL_EXTERNAL_PLUGINS" nil nil newline)
     (:reveal-default-frag-style "REVEAL_DEFAULT_FRAG_STYLE" nil org-reveal-default-frag-style t)
     (:reveal-single-file nil "reveal_single_file" org-reveal-single-file t)
     (:reveal-init-script "REVEAL_INIT_SCRIPT" nil org-reveal-init-script space)
@@ -763,8 +764,15 @@ dependencies: [
                    (or (and buffer-plugins (listp buffer-plugins) buffer-plugins)
                        org-reveal-plugins))))
                (external-plugins
-                (cl-loop for (key . value) in org-reveal-external-plugins
-                         collect (format  value root-path )) )
+		(append
+		 ;; Global setting
+                 (cl-loop for (key . value) in org-reveal-external-plugins
+                          collect (format  value root-path ))
+		 ;; Local settings
+		 (list (let ((local-plugins (plist-get info :reveal-external-plugins)))
+			 (and local-plugins
+			      (format local-plugins root-path))))))
+
                (all-plugins (if external-plugins (append external-plugins builtin-codes) builtin-codes))
                (extra-codes (plist-get info :reveal-extra-js))
                (total-codes