Allow fields to have arrays of associated items.

Allow you do do things like

```js
  Meteor.publishWithRelations({
    handle: this,
    collection: Loops,
    mappings: [{
      collection: Profiles,
      key: 'profileIds'
    }]
  }
```

Where a loop looks like:

```js
{_id: 'abcd', profileIds: ['xcvb', 'cvbb']}
```