Description

The IssueEvent in issues_event.go file doesn't have the action key that defines the kind of action being performed by the webhook API. Adding this key would give a more robust and rich understanding of the actions themselves.

An examples webhook API request is as follows that was triggered when an issue was edited :

{
  "action": "edited",
  "issue": {
    // omitting the nested-json for brevity  
  },
  "repository": {
    // omitting the nested-json for brevity  
  },
  "sender": {
    // omitting the nested-json for brevity
  }
}

I can also push attach a PR to add this as well some more useful features around the same. Let me know if that is welcomed by the maintainers :)