Allowed joins to nest

- The nesting is determined from the `leftKeys`. For example, if you have `leftKeys = ['comments.userId', 'comments.userName']`, then the join data will nested under 'comments' under the newKey that you choose.
Just realised there is a mistake here. This should more like: if `theObject` is an array, then insert at position `i`, otherwise set a key-value pair on theObject as rightRecord:

````
if (Array.isArray(theObject)) {
  theObject[i][joinArgs.newKey] = rightRecord;
} else {
  theObject[joinArgs.newKey] = rightRecord;
}
````