Feature: Add support for conditional slack notifications

[jjpatel361]

Use case

It would be great to add built-in support to conditionally send slack notifications. One use case is, send notification only for specific deploys Prod / QA and not dev or feature branches.

Suggested

custom:
  notification:
    deployer: /* Default deployer name*/
    slack:
      enabled: {{self:provider.stage}}

enabled flag is true we perform notification.

Since sendNotification is the sink of all promises might as well put it there. or skip the entire promise chaining.

Also thanks for this plugin. 🙏

[hieuunguyeen]

Hi 👋 I haven't been actively maintain this plugin. If you could open a PR for it I'll happy to take!