Allow user to pass paramaters when fetching invoices

This allows users to pass more information to the invoices request. I had a need for this in a project I was working on so I figured it'd be good to add. I'm using it to get information about the charge and card used to pay an invoice.

For example you could do something like this to get the last 4 of the card used.

```
foreach ($customer->invoices(['expand' => ['data.charge']]) as $invoice) {
    echo $invoice->charge->card->last4;
}
```