Resolve Issue #1188 (Allow Users to specify position when creating a new channel) (#1196)

* Added ability to specify position when creating a channel

* Adjusted categories to include guildproperties and allow specifying position when creating channel categories

* fixed unimplemented methods (for CreateCategoryChannelAsync) and added appropriate documentation

src/Discord.Net.Rest/API/Rest/CreateGuildChannelParams.cs

@@ -12,6 +12,8 @@ namespace Discord.API.Rest
         public ChannelType Type { get; }
         [JsonProperty("parent_id")]
         public Optional<ulong?> CategoryId { get; set; }
+        [JsonProperty("position")]
+        public Optional<int> Position { get; set; }
 
         //Text channels
         [JsonProperty("topic")]