[13.x] Create invoices separately

[driesvints]

This PR adds some new functionality to Cashier to allow to create invoices separately without sending them (draft state) as well as a few methods to add new items to invoices directly.

// Creates a new empty draft invoice...
$invoice = $user->createInvoice();

// Add a new price line item to the invoice...
$invoice->tabPrice('price_xxx', 5);

Closes #1397