[7.x] Add Mailgun and Postmark mailer (#5243)

* Add Mailgun and Postmark mailer

* Formatting

* Update mail.php

Co-authored-by: Taylor Otwell <taylor@laravel.com>

config/mail.php

@@ -47,6 +47,14 @@ return [
             'transport' => 'ses',
         ],
 
+        'mailgun' => [
+            'transport' => 'mailgun',
+        ],
+
+        'postmark' => [
+            'transport' => 'postmark',
+        ],
+
         'sendmail' => [
             'transport' => 'sendmail',
             'path' => '/usr/sbin/sendmail -bs',