Nobody expected Lily & Madeleine’s first original song, “In the Middle,” to rack up a quarter of a million YouTube views. The Internet is obsessed with what’s new and what’s next, the fads and memes of viral culture, and so it would have to seem a little incongruous for social networks and content aggregators to embrace gimmick-free, black-and-white footage of an unplugged duo crooning a bittersweet and understated melody.