Lululemon shares tumbled on its second-quarter earnings report. Is this a buying opportunity?Lululemon Athletica's (LULU 0.12%) dismal year just got even worse.The stock tumbled 18.6% on Sept. 5 after it once again missed estimates and cut its full-year guidance due to weakness in the U.S. as it faces a range of challenges, some short-term and others more structural. The stock is now down 56% this year, a remarkable collapse for what has been one of the best-performing apparel stocks of the last 20 years.Re ...