Echo extraneous output after prompt has been received

When the REPL spits out more stdout when no command is (ostensibly) in
progress, echo that output before the prompt.

Example case:

λ> forkIO (do threadDelay (1000*1000); putStrLn "yo")
ThreadId 37

yo
λ>

The "yo" is usually just lost in the ether. It also doesn't disturb
your prompt if you're typing in it.

@hvr @cmears might be interested.