The RompHim clan ain’t the only onesie-wearing trendsetters. Yesterday (May 24), Lauren Alaina held a “Number Onesie” party at Nashville’s Sutler Saloon to celebrate the No. 1 success of her hit single, “Road Less Traveled.” Many of the revelers—including Lauren and the song’s co-writers, Meghan Trainor and Jesse Frasure—were decked out in custom onesies. “I…