Add extension method for subscriptions

Tests/MQTTnet.TestApp.UniversalWindows/MainPage.xaml

@@ -19,6 +19,8 @@
                 <StackPanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
                     <TextBlock>Server:</TextBlock>
                     <TextBox x:Name="Server"></TextBox>
+                    <TextBlock>Port:</TextBlock>
+                    <TextBox x:Name="Port" Text="1883"></TextBox>
                     <TextBlock>User:</TextBlock>
                     <TextBox x:Name="User"></TextBox>
                     <TextBlock>Password:</TextBlock>

Tests/MQTTnet.TestApp.UniversalWindows/MainPage.xaml.cs

@@ -78,6 +78,7 @@ namespace MQTTnet.TestApp.UniversalWindows
                 options.ChannelOptions = new MqttClientTcpOptions
                 {
                     Server = Server.Text,
+                    Port = int.Parse(Port.Text),
                     TlsOptions = tlsOptions
                 };
             }