multi-line expressions (sort of)

if you type this:
`> (define (f x)`
and then hit enter, it will show a new prompt.
Then you can finish the expression after that:
`> (define (f x)
>   x)
> (f 1)
1`
Merge pull request #2 from AlexKnauth/try-muli-line

multi-line expressions (sort of)