Add SyncPermissionsAsync to Sync Child Channels with its Parent (#1159)

* Initial implementation

* Adjust according to comments

See: 6e76b45713 (diff-58466c35787d448266d026692e467baa)

src/Discord.Net.Core/Entities/Channels/INestedChannel.cs

@@ -25,5 +25,10 @@ namespace Discord
         ///     representing the parent of this channel; <c>null</c> if none is set.
         /// </returns>
         Task<ICategoryChannel> GetCategoryAsync(CacheMode mode = CacheMode.AllowDownload, RequestOptions options = null);
+
+        /// <summary>
+        ///     Syncs the permissions of this nested channel with its parent's.
+        /// </summary>
+        Task SyncPermissionsAsync(RequestOptions options = null);
     }
 }