Frankly, think can be easily done without changing the REPL interface. Just don't call `repl.start()` until the "end" callback.

I'm -1 on changes to `lib/repl.js`, but maybe this would be the time to move the main repl stuff into a private `lib/_main_repl.js` module or something, since that code is starting to become a lot for `src/node.js` IMO.