Add ability to send push to channel tag

Author: Alexey Plekhanov

README.md

@@ -82,8 +82,18 @@ public function routeNotificationForPushbullet()
 }
 ```
 
-#### Third option:
-Although, this option is not recommended, you might just return a string (email or device id) and library will do its best to determine if it email or device id.
+#### Sending notification to all subscribers of channel that has tag:
+```php
+public function routeNotificationForPushbullet()
+{
+    return new \NotificationChannels\Pushbullet\Targets\Channel($this->channel_tag);
+}
+```
+
+#### Fourth option (**deprecated**):
+Although, this option is not recommended, you might just return a string (email or device id) and library will do its best to determine if it is email or device id.
+
+**Warning!** This option doesn't give you possibility to have channel tag guessed, only email or device id.
 ```php
 public function routeNotificationForPushbullet()
 {

src/Targets/Channel.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace NotificationChannels\Pushbullet\Targets;
+
+class Channel implements Targetable
+{
+    /**
+     * PushBullet channel tag
+     *
+     * @var string
+     */
+    protected $channelTag;
+
+    /**
+     * Set channel tag
+     *
+     * @param  string  $channelTag
+     */
+    public function __construct($channelTag)
+    {
+        $this->channelTag = $channelTag;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getTarget()
+    {
+        return ['channel_tag' => $this->channelTag];
+    }
+}