[13.x] Add invoicePrice and tabPrice methods

[driesvints]

Adds an invoicePrice and tabPrice method that works with a given Price ID. This is a useful alternative to single charge invoices which cannot be used in combination with automatic tax calculation.

// Invoice for a single price...
$invoice = $user->invoicePrice('price_tshirt', 5);

// Invoice for a couple of prices
$user->tabPrice('price_tshirt', 5);
$user->tabPrice('price_mug', 5);
$invoice = $user->invoice();

Closes #1211