Enhancement: `to_dict` method

Hi there,

Thanks for this library. I'd like to ask for a convenience method that output a dict of a multidict - with lists for multi-values. Currently I have this:

```python
def dict(self) -> Dict[str, List[Any]]:
"""

Returns:
A dict of lists
"""
return {k: self.getall(k) for k in set(self.keys)}
```

While this works, its not very performant.