Added GetJumpUrl() as an IMessage extension method

src/Discord.Net.Commands/Extensions/MessageExtensions.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 
 namespace Discord.Commands
 {
@@ -42,5 +42,11 @@ namespace Discord.Commands
             }
             return false;
         }
+
+        public static string GetJumpUrl(this IMessage msg)
+        {
+            var channel = msg.Channel;
+            return $"https://discordapp.com/channels/{(channel is IDMChannel ? "@me" : $"{(channel as IGuildChannel).GuildId}")}/{channel.Id}/{msg.Id}";
+        }
     }
-}
+}