babel: Declare R-specific header arguments

* babel/lisp/langs/org-babel-R.el
(org-babel-header-arg-names:R):
New variable to contain R-specific header args

contrib/babel/lisp/langs/org-babel-R.el

@@ -35,6 +35,12 @@
 
 (add-to-list 'org-babel-tangle-langs '("R" "R" "#!/usr/bin/env Rscript"))
 
+(defconst org-babel-header-arg-names:R
+  '(width height bg units pointsize antialias quality compression
+	  res type family title fonts version paper encoding
+	  pagecentre colormodel useDingbats horizontal)
+  "R-specific header arguments.")
+
 (defun org-babel-expand-body:R (body params &optional processed-params)
   (let* ((processed-params (or processed-params
                                (org-babel-process-params params)))