The haunting folk melodies of the Civil Wars have been praised by everyone from T. Swift to Adele; likened to Alison Krauss-Robert Plant duets; and—of course—featured on Grey’s Anatomy, now a rite of passage for any indie band. Hear the Nashville duo before they get too big for cozy venues. They play two nights at the Triple Door, tonight and Thursday; shows start at 7:30, and tickets ($15) are selling quickly.