Feature: Add Events API support

[Corkle]

The current struct for Slack data returned from the plug is intended to be generically compatible with both Slack Commands and Interactive Messages. The Slack data should be set to private params with more specific data shape based on the incoming request type. This change will make a more flexible library to add new features like the Events API which introduces new fields based on the event type.

TODO:

1. Refactor Slack Plug with new structs and private param return values
2. Add support for Events API request