Support stdin input in the REPL

@hvr

λ> do putStr "Enter your name: "; line <- getLine; putStr "Enter your age: "; line2 <- getLine; putStrLn ("Your name is: " ++ line ++ " and age " ++ line2 ++ "!")
Enter your name: Chris
Enter your age: 42
Your name is: Chris and age 42!