Added GetChannels extension to WebSocket->IGuild

Adds a sync method to retrieve all cached IGuildChannels on an IGuild.

src/Discord.Net/WebSocket/Extensions/GuildExtensions.cs

@@ -12,6 +12,9 @@ namespace Discord.WebSocket.Extensions
         public static IGuildChannel GetChannel(this IGuild guild, ulong id) =>
             GetSocketGuild(guild).GetChannel(id);
 
+        public static IEnumerable<IGuildChannel> GetChannels(this IGuild guild) =>
+            GetSocketGuild(guild).Channels;
+
         public static ITextChannel GetTextChannel(this IGuild guild, ulong id) =>
             GetSocketGuild(guild).GetChannel(id) as ITextChannel;