Add support for inline price data

[josiasmontag]

Stripe allows to create subscriptions and invoices without creating a price object via the dashboard beforehand. This is useful in cases where you want to create very individual subscriptions or invoices.

This PR adds the possibility to subscribe, swap and invoice with custom price_data:

// Create Subscription

$user->newSubscription('main')->price([
    'price_data' => [
        'product'      => $productId,
        'tax_behavior' => 'exclusive',
        'currency'     => 'USD',
        'recurring'    => [
            'interval' => 'month',
        ],
        'unit_amount'  => 1100
    ]
])
    ->create('pm_card_visa');

// Create Invoice

$user->invoiceFor('Laravel T-shirt', 599, [
    'price_data' => [
        'product'      => $productId,
        'tax_behavior' => 'exclusive'
    ]
]);

// Swap Subscription

$subscription->swap([
    [
        'price_data' => [
            'product'      => $productId,
            'tax_behavior' => 'exclusive',
            'currency'     => 'USD',
            'recurring'    => [
                'interval' => 'month',
            ],
            'unit_amount'  => 1100
        ]
    ]
]);

This also fixes one-off charges when automatic tax is enabled (#1204)

[driesvints]

Thanks for the great PR @josiasmontag 👍