Added GetJumpUrl() as an extension method for IMessage (#1102)

* Added GetJumpUrl() as an IMessage extension method

* Removed extra line for consistency

* Moved the namespace from Discord.Commands to Discord

* lint: remove eof newline

* lint: use TextChannel to get GuildID

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

@@ -1,4 +1,4 @@
-using System;
+using System;
 
 namespace Discord.Commands
 {
@@ -43,4 +43,4 @@ namespace Discord.Commands
             return false;
         }
     }
-}
+}

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

@@ -0,0 +1,11 @@
+namespace Discord
+{
+    public static class MessageExtensions
+    {
+        public static string GetJumpUrl(this IMessage msg)
+        {
+            var channel = msg.Channel;
+            return $"https://discordapp.com/channels/{(channel is IDMChannel ? "@me" : $"{(channel as ITextChannel).GuildId}")}/{channel.Id}/{msg.Id}";
+        }
+    }
+}