Allowance for webhookUrl parameter.

The Mollie API has a webhookUrl parameter. This allows to change the callback that Mollie does after a successful payment. The default hook can be set in the Mollie admin panel. Test is included.

For more info, please see.
https://www.mollie.nl/files/documentatie/payments-api.html
the trailing comma is usually added on purpose, to make it easier to add a new line.