Org writer: ensure blank line after figure

An Org-mode figure should be surrounded by blank lines.  The figure
would be recognized regardless, but images in the following line would
unintentionally be treated as figures as well.

src/Text/Pandoc/Writers/Org.hs

@@ -160,7 +160,7 @@ blockToOrg (Para [Image attr txt (src,'f':'i':'g':':':tit)]) = do
              then return empty
              else ("#+CAPTION: " <>) `fmap` inlineListToOrg txt
   img <- inlineToOrg (Image attr txt (src,tit))
-  return $ capt $$ img
+  return $ capt $$ img $$ blankline
 blockToOrg (Para inlines) = do
   contents <- inlineListToOrg inlines
   return $ contents <> blankline

tests/writer.org

@@ -809,6 +809,7 @@ From "Voyage dans la Lune" by Georges Melies (1902):
 
 #+CAPTION: lalune
 [[lalune.jpg]]
+
 Here is a movie [[movie.jpg]] icon.
 
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