feature: Add GuildEmote#CreatorID (#1213)

Co-authored-by: Chris Johnston <chris@thejohnstons.net>

commit 23f5abba4822151a6ba9cb5078752b113c4b8ce6
Author: Christopher Felegy <foxbot@protonmail.com>
Date:   Thu Dec 20 17:10:21 2018 -0500

    lint: clean up some long lines

commit 4a8a809db0c2d70c61367c3e1ed4013107cf9f0e
Author: Chris Johnston <chris@thejohnstons.net>
Date:   Sat Dec 15 00:33:05 2018 -0800

    Explain why CreatorId can be null sometimes

commit 9442e4e6358f85177e356f88cd82b73f9f42b1bb
Author: Chris Johnston <chris@thejohnstons.net>
Date:   Fri Dec 14 23:59:01 2018 -0800

    Add the CreatorId property to GuildEmote implementation

commit e0eb94d44cb37c259c27998878330bd68e5706c4
Author: Chris Johnston <chris@thejohnstons.net>
Date:   Fri Dec 14 23:41:54 2018 -0800

    Update the Emoji API model to add User? attribute

src/Discord.Net.Rest/API/Common/Emoji.cs

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 
 namespace Discord.API
@@ -17,5 +17,7 @@ namespace Discord.API
         public bool RequireColons { get; set; }
         [JsonProperty("managed")]
         public bool Managed { get; set; }
+        [JsonProperty("user")]
+        public Optional<User> User { get; set; }
     }
 }