add support for slack notification

[go-kal]

feature enhancement: #14

Based on deployment status, sends a slack notification based on the pre-configured webhook.

[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 😄