Fix builtin commands handling in the linker plugin

linker/linker.py

@@ -109,7 +109,7 @@ class Linkers(object):
             link = Link(context, content_object, match)
 
             if link.cmd in LinkerBase.builtins:
-                pass # builtin commands not handled here
+                return match.group(0)  # builtin commands not handled here
             elif link.cmd in self.linkers:
                 self.linkers[link.cmd].link(link) # let Linker process the Link
             else: