Slack notification (Closes #200)

Add an [Incoming Webhook](https://<team>.slack.com/services/new/incoming-webhook) and get token for integration.

```yaml
notify:
  slack:
    team: {{slackTeam}}
    channel: {{slackChannel}}
    username: drone
    token: {{slackToken}}
    on_started: true
    on_failure: true
    on_success: true
```