Add overload for subscribing.

Build/MQTTnet.nuspec

@@ -19,6 +19,7 @@
 * [Client] Fixed an issue in _ManagedClient_ which can cause the client to stop when publishing subscriptions.
 * [Client] Fixed an issue in _ManagedClient_ which prevents changing the QoS of an existing subscription (BREAKING CHANGE!).
 * [Client] Fixed an issue in _MqttClientOptionsBuilder_ which prevents adding TLS options to the client options when calling _Build()_. (Thanks to @cvellan).
+* [Client] Added an overload for subscribing at both clients which reduces required code.
 * [Server] The application message interceptor can now delete any received application message.
 * [Server] Added a ConnectionValidator context to align with other APIs (BREAKING CHANGE! Please find new example in Wiki).
 * [Server] Added an interface for the _MqttServerOptions_.

Frameworks/MQTTnet.NetStandard/ManagedClient/ManagedMqttClientExtensions.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Protocol;
 
 namespace MQTTnet.ManagedClient
 {
@@ -12,7 +13,15 @@ namespace MQTTnet.ManagedClient
             return managedClient.SubscribeAsync(topicFilters);
         }
 
-        public static Task UnsubscribeAsync(this IManagedMqttClient managedClient, params TopicFilter[] topicFilters)
+        public static Task SubscribeAsync(this IManagedMqttClient managedClient, string topic, MqttQualityOfServiceLevel qualityOfServiceLevel)
+        {
+            if (managedClient == null) throw new ArgumentNullException(nameof(managedClient));
+            if (topic == null) throw new ArgumentNullException(nameof(topic));
+
+            return managedClient.SubscribeAsync(new TopicFilterBuilder().WithTopic(topic).WithQualityOfServiceLevel(qualityOfServiceLevel).Build());
+        }
+
+        public static Task UnsubscribeAsync(this IManagedMqttClient managedClient, params string[] topicFilters)
         {
             if (managedClient == null) throw new ArgumentNullException(nameof(managedClient));