Siphiwe Sibeko/File Photo STELLENBOSCH, South Africa (Reuters) - A budget bank is booming in South Africa’s economic slump, challenging the decades-long dominance of the “big four” lenders and prompting a price war that is driving down banking costs in a country where many people can’t afford an account. Capitec Bank has doubled its customer numbers over the past five years and quadrupled in market value, even as South Africa’s economic growth has stalled and the country has slid into...