Adds Stripe payment processing to Checkout.
$conf = [
'modules' => [
'stripe' => [
// A thankyou email message id that is sent to the buyer on
// successful purchase.
'buyer_email' => {int}
// An alert message to send to the seller when an order
// has been made.
'seller_email' => {int}
// The descriptoer to appear on the credit card statement
// of the buyer
'statement_descriptor' => {string}
// Whether to allow bitcoin payment processing.
'bitcoin' => false,
// Public key for processing transactions
'public' => {string},
// Private key for processing transactions
'private' => {string},
]
]
];