Bitcoin's run-up has grown famous. Late Thursday morning, the digital currency was trading for more than $19,000 on exchanges in South Korea, before closing around $16,600 on Bitfinex, a large cryptocurrency exchange. The $19,000-ish price was up more than $5,000 in less than 24 hours. It was up from $12,000 at the start of the week. And it was up from a little more than $1,000 on Jan. 1.