Trying to remember that pithy, brilliantly composed tweet about the latest Wes Anderson movie that you fired off a few months ago? You’re out of luck: Twitter gives users access only to the last few thousand posts made to the site. But Dick Costolo, Twitter’s chief executive, promises that this will eventually change. “We’re working on a tool to let users export all of their tweets,” Mr. Costolo said in a meeting with reporters and editors at The New York Times on Monday. “You’ll be able to download a file of them.”