Issue title: Notification to slack
 Issue body: Can you add notification to slack
 Comments: 
 Comment 0: @programmermarvin, Yes, there is a possibility to add this feature. But my time is limited and I will try to do it as soon as possible. 
If you can to help expand this package, you can do the following.

Fork, then clone the repo:
```bash
git clone mevans@example.org:your-username/laravel-social-auto-posting.git
```
Make your changes and push to your fork and submit a pull request.

Thanks for your interest.

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