api: add slow mode

This adds the following property to ITextChannel
- SlowMode (int)

The SlowMode field defines the time in seconds users must wait between
sending messages; if zero, slow-mode is disabled.

Bots are not affected by slow-mode, and as such, no changes need to be
made to the REST client's internal ratelimiting. This is strictly a
read/write cosmetic property for bots.

src/Discord.Net.WebSocket/Entities/Channels/SocketTextChannel.cs

@@ -16,6 +16,7 @@ namespace Discord.WebSocket
         private readonly MessageCache _messages;
 
         public string Topic { get; private set; }
+        public int SlowMode { get; private set; }
         public ulong? CategoryId { get; private set; }
         public ICategoryChannel Category
             => CategoryId.HasValue ? Guild.GetChannel(CategoryId.Value) as ICategoryChannel : null;
@@ -47,6 +48,7 @@ namespace Discord.WebSocket
             base.Update(state, model);
             CategoryId = model.CategoryId;
             Topic = model.Topic.Value;
+            SlowMode = model.SlowMode.GetValueOrDefault(); // some guilds haven't been patched to include this yet?
             _nsfw = model.Nsfw.GetValueOrDefault();
         }