增加mailgun默认配置项

.env.example

@@ -35,6 +35,8 @@ MAIL_PASSWORD=password
 MAIL_ENCRYPTION=ssl
 [email protected]
 MAIL_FROM_NAME=SSRPanel
+MAILGUN_DOMAIN=
+MAILGUN_SECRET=
 
 PUSHER_APP_ID=
 PUSHER_APP_KEY=
@@ -44,4 +46,4 @@ PUSHER_APP_CLUSTER=mt1
 MIX_PUSHER_APP_KEY="${PUSHER_APP_KEY}"
 MIX_PUSHER_APP_CLUSTER="${PUSHER_APP_CLUSTER}"
 
-REDIRECT_HTTPS=false
+REDIRECT_HTTPS=false