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.Rest/Entities/Channels/RestTextChannel.cs

@@ -12,6 +12,7 @@ namespace Discord.Rest
     public class RestTextChannel : RestGuildChannel, IRestMessageChannel, ITextChannel
     {
         public string Topic { get; private set; }
+        public int SlowMode { get; private set; } 
         public ulong? CategoryId { get; private set; }
 
         public string Mention => MentionUtils.MentionChannel(Id);
@@ -34,6 +35,7 @@ namespace Discord.Rest
             base.Update(model);
             CategoryId = model.CategoryId;
             Topic = model.Topic.Value;
+            SlowMode = model.SlowMode.Value;
             _nsfw = model.Nsfw.GetValueOrDefault();
         }