Add label activity event.

[BURG3R5]

Labels are helpful tags which users can use to sort and manage issues/PRs for their GitHub repository. From the documentation:

You can manage your work on GitHub by creating labels to categorize issues and pull requests. You can apply labels in the repository the label was created in. Once a label exists, you can use the label on any issue or pull request within that repository.

This issue consists of adding a new event for the bot to track. You'll be updating the bot to make sure that users can be notified when labels are added/modified/deleted in their repository.

These are the steps to do so:

• [ ] #7
• [ ] #8
• [ ] #9
• [ ] #10

More information can be found in the individual issues. Leave a comment if you have any doubts.