Add $queue attribute

This code enables me to change the queue name that the JobPipeline will be pushed to.

Now I can do something like this:
```php
JobPipeline::make([
    Jobs\CreateDatabase::class,
    Jobs\MigrateDatabase::class,
    // Jobs\SeedDatabase::class,

    // Your own jobs to prepare the tenant.
    // Provision API keys, create S3 buckets, anything you want!

])->send(function (Events\TenantCreated $event)
{
    return $event->tenant;
})->onQueue('another-queue')
    ->shouldBeQueued()
```

src/JobPipeline.php

@@ -26,6 +26,9 @@ class JobPipeline implements ShouldQueue
 
     /** @var bool */
     public $shouldBeQueued;
+    
+    /** @var string */
+    public $queue;
 
     public function __construct($jobs, callable $send = null, bool $shouldBeQueued = null)
     {
@@ -56,6 +59,13 @@ class JobPipeline implements ShouldQueue
 
         return $this;
     }
+    
+    public function onQueue(string $queue)
+    {
+        $this->queue = $queue;
+
+        return $this;
+    }
 
     public function handle(): void
     {