Create Artisan command that makes Slack event listeners 

I think this package also should provide artisan command like:

``` bash
php artisan make:slackListener
```

Possible syntax example:

```
php artisan make:slackListener ListenerClassName --event=slack_event_name
```

Real-world syntax example:

```
php artisan make:slackListener ReactionAddedListener --event=reaction_added
```
