added sender_address to switfmailer provider

doc/providers/swiftmailer.rst

@@ -12,9 +12,13 @@ Parameters
 
 * **swiftmailer.use_spool**: A boolean to specify whether or not to use the
   memory spool, defaults to true. 
+
 * **swiftmailer.options**: An array of options for the default SMTP-based
   configuration.
 
+* **swiftmailer.sender_address**: If set, all messages will be delivered with
+  this address as the "return path" address.
+
   The following options can be set:
 
   * **host**: SMTP hostname, defaults to 'localhost'.

src/Silex/Provider/SwiftmailerServiceProvider.php

@@ -71,6 +71,10 @@ class SwiftmailerServiceProvider implements ServiceProviderInterface, EventListe
             $transport->setPassword($options['password']);
             $transport->setAuthMode($options['auth_mode']);
 
+            if (null !== $app['swiftmailer.sender_address']) {
+                $transport->registerPlugin(new \Swift_Plugins_ImpersonatePlugin($app['swiftmailer.sender_address']));
+            }
+
             return $transport;
         };
 
@@ -89,6 +93,8 @@ class SwiftmailerServiceProvider implements ServiceProviderInterface, EventListe
         $app['swiftmailer.transport.eventdispatcher'] = function () {
             return new \Swift_Events_SimpleEventDispatcher();
         };
+
+        $app['swiftmailer.sender_address'] = null;
     }
 
     public function subscribe(Container $app, EventDispatcherInterface $dispatcher)