oneToMany support in _.invert

A tiny feature request to support inverting objects that may have multiple keys with the same value. By adding an optional `oneToMany` parameter to `_.invert()`. 

```javascript
_.invert({ 'first': 'fred', 'second': 'barney', 'third': 'fred' }, true);
// => { 'fred': ['first', 'third'], 'barney': ['second'] }
```

Thanks. Beautiful library.