By Martyn Herman RIO DE JANEIRO, Aug 11 (Reuters) - Britain's Mark Cavendish will be a major contender for the Olympic omnium after his outstanding stage victories in last month's Tour de France, medal rival Roger Kluge believes. Germany's Kluge took the silver medal in the world championships in London this year when Cavendish struggled and finished sixth, but he expects the Briton to mount a much stronger challenge in Rio. Cavendish, like Kluge, has been juggling his professional road career with track training this year, and arrived in Brazil on the back of winning four stages on the Tour de France, increasing his overall tally to 30. More...