(adds detail) By Martyn Herman RIO DE JANEIRO, Aug 15 (Reuters) - Italy's Elia Viviani recovered from a mid-race crash to win the gold medal in the track cycling men's omnium on Monday after a thrilling finish in the Olympic velodrome. An attritional battle in the 160-lap points race -- the final event of the two-day omnium -- ended with Viviani doing enough to hold off his main rivals for the title. After winning the penultimate sprint, Viviani knew the gold was all but secure unless either Mark Cavendish or 2012 champion Lasse Norman Hansen managed to launch an audacious late attack. More...