enh: add support for Slack notification

[go-kal]

This PR is an enhancement to "add support for Slack Notification" #14

Please review the changes and let me know if any changes are required.

[go-kal]

Happy path:

• cfn-test user has all the required permissions.

• Build SUCCESSFUL 👍

• Slack notification 😀

Failure scenario:

• cfn-test user does not have the required permissions. Access to CloudFormation is removed.

• Build FAILED 👎

• Slack notification 😄