Interactive-Haskell multiline support

By default, `Interactive-Haskell` change `prompt`, but not `prompt2`, so it looks strange, when try to use multiline input:

``` haskell
λ> :set +m
λ> let
*Main FunnyPrint Main|
λ>
```

Start with the Interactive-Haskell handling of `prompt` and `prompt2`, then reproduce the example using `:set +m` and a multiline `let`. Check how the two prompts are selected during multiline input; done means the multiline prompt no longer looks inconsistent with the regular prompt.

Written by the indexing model from the issue text.