People like to feel pretty. Over the past ten years Americans have spent more than $500bn on beauty products. By 2024 the Chinese are expected to splurge more than $100bn a year. In the past most of that would have gone to cosmetics conglomerates, such as L’Oréal and Estée Lauder, or to consumer-products giants like Unilever, selling every imaginable tincture to make everything from toenails to tresses more fetching. But in recent years…