Add filter method on ManyToMany join table.

Is there any way to add `filter` options on ManyToMany `through`?

like,

```js
goodFriends: {
  relation: ManyToManyRelation,
  to: XX,
  from: YY,
  through: {
    from: 'XX.id'
    to: 'YY.id',
    filter: (builder) => { builder.where('relationships.type', 'good') }
  }
}
```