feat(message): allow sending driver extras

`driverExtras` method can be used to send extras to the current driver in use

src/Mail/Message/index.js

@@ -391,6 +391,22 @@ class Message {
     return this.attach(filePath, Object.assign({ cid }, options))
   }
 
+  /**
+   * Set extras to be sent to the current driver in
+   * use. It is the responsibility of the driver
+   * to parse and use the extras
+   *
+   * @method driverExtras
+   *
+   * @param  {Object}     extras
+   *
+   * @chainable
+   */
+  driverExtras (extras) {
+    this.mailerMessage.extras = extras
+    return this
+  }
+
   /**
    * Returns nodemailer compatable message
    * object

test/message.spec.js

@@ -199,4 +199,13 @@ test.group('Message', () => {
       inReplyTo: '11010'
     })
   })
+
+  test('define driver extras', (assert) => {
+    const message = new Message()
+    message.driverExtras({ campaign_id: 20 })
+
+    assert.deepEqual(message.toJSON(), {
+      extras: { campaign_id: 20 }
+    })
+  })
 })