Add IUser#SendMessageAsync extension (#706)

* Add IUser#SendMessageAsync extension

* Add ConfigureAwait

src/Discord.Net.Core/Extensions/UserExtensions.cs

@@ -0,0 +1,16 @@
+using System.Threading.Tasks;
+
+namespace Discord
+{
+    public static class UserExtensions
+    {
+        public static async Task<IUserMessage> SendMessageAsync(this IUser user, 
+            string text, 
+            bool isTTS = false,
+            Embed embed = null, 
+            RequestOptions options = null)
+        {
+            return await (await user.GetOrCreateDMChannelAsync().ConfigureAwait(false)).SendMessageAsync(text, isTTS, embed, options).ConfigureAwait(false);
+        }
+    }
+}