Feature request: Multiline input for eval plugin

I have tried to get multiline inputs working for the eval plugin however to no avail. What I would like is support for the following syntax:

```haskell
>>> tree = :{ Node 9 [
>>> Leaf 5,
>>> Leaft 12
>>> ] :}
-- or
>>> :set +m
>>> tree = Node 9 [
>>> Leaf 5,
>>> Leaft 12
>>> ]
```

However this always results in an exception like `parse error on input ‘:’` and `parse error (possibly incorrect indentation or mismatched brackets)` respectively.