Feature Requests Allow custom headers in webhook requests

It would be great if you could sent custom headers (and values) in webhook requests. This would allow you to add an API-key as a header which you can check in the system receiving the webhooks to validate it is actually the updown-io services sending the request.

Currently this is do-able by adding a GET-parameter with a key to the webhook URL, but sending custom headers would be a more preferable option.