Support many-to-many relationships

Nobody has claimed this yet.

It seems that I can't do this right now:

```js
module.exports = {
posts: [
{ id: 1, title: "Lorem Ipsum", views: 254, user_id: [123, 456] },
{ id: 2, title: "Sic Dolor amet", views: 65, user_id: [456, 457] },
],
users: [
{ id: 123, name: "John Doe" },
{ id: 456, name: "Jane Doe" },
{ id: 457, name: "Xavier" }
],
comments: [
{ id: 987, post_id: 1, body: "Consectetur adipiscing elit", date: new Date('2017-07-03') },
{ id: 995, post_id: 1, body: "Nam molestie pellentesque dui", date: new Date('2017-08-17') }
]
}
```

Is this not supported? or maybe there's a different way of doing it?

p.s.: this would be something like a combined authorship post. This is not my actual use case though, but close enough to show the problem and with a little change of the provided example.

p.s. 2: great tool, keep it up :)

Start by reproducing the relationship shown in the issue body against the repository's generated GraphQL API and compare it with the existing relationship examples, if any. Determine whether many-to-many relationships are intended to be supported or documented as unsupported; done means the expected behavior is implemented and verified, or the limitation and alternative usage are clearly documented.

Written by the indexing model from the issue text.