Implement `filter` option

Webhooks can become noisy, especially org-level webhooks, e.g. for projects. If your app handles the `projects_v2_item` event, and your org has a lot of projects with a lot of usage, you will have a very hard time to spot the events relevant to you.

Adding a `filter` option would help in this case. It could be a faction that is called with the webhook event object. If it returns with a truthy value, the event will be forwarded to the app, otherwise not.