A `WebhookEvent` hook can now return a tuple in the format `(data,
http_code, headers)` in order to customize the HTTP status code and the
headers of a response.
When a client triggers a `WebhookEvent` by calling a configured webhook
over `/hook/<hook_name>`, the server will now wait for the configured
`@hook` function to complete and it will return the returned response
back to the client.
This makes webhooks much more powerful, as they can be used to proxy
HTTP calls or other services, and in general return something to the
client instead of just executing actions.
As the Mercury reader web API is deprecated, and the only available
implementation is the open source mercury-parser, node, npm and
@postlight/mercury-parser have to be added as dependencies for the
http.webpage plugin (or at least for the `simplify` action).