Add Extension Methods for WebSocket->IGuild

For users importing `Discord.WebSocket.Extensions`, there are now non-async extensions for Get(Type)Channel(s) and Get(Current)User(s).

These methods point to the appropriate method or member on SocketGuild.

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

@@ -0,0 +1,44 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Discord.WebSocket.Extensions
+{
+    // TODO: Docstrings
+    public static class GuildExtensions
+    {
+        // Channels
+
+        public static IGuildChannel GetChannel(this IGuild guild, ulong id) =>
+            (guild as SocketGuild).GetChannel(id);
+
+        public static ITextChannel GetTextChannel(this IGuild guild, ulong id) =>
+            (guild as SocketGuild).GetChannel(id) as ITextChannel;
+
+        public static IEnumerable<ITextChannel> GetTextChannels(this IGuild guild) =>
+            (guild as SocketGuild).Channels.Select(c => c as ITextChannel).Where(c => c != null);
+
+
+        public static IVoiceChannel GetVoiceChannel(this IGuild guild, ulong id) =>
+            (guild as SocketGuild).GetChannel(id) as IVoiceChannel;
+
+        public static IEnumerable<IVoiceChannel> GetVoiceChannels(this IGuild guild) =>
+            (guild as SocketGuild).Channels.Select(c => c as IVoiceChannel).Where(c => c != null);
+
+        // Users
+
+        public static IGuildUser GetCurrentUser(this IGuild guild) =>
+            (guild as SocketGuild).CurrentUser;
+
+        public static IGuildUser GetUser(this IGuild guild, ulong id) =>
+            (guild as SocketGuild).GetUser(id);
+
+        public static IEnumerable<IGuildUser> GetUsers(this IGuild guild) =>
+            (guild as SocketGuild).Members;
+
+        public static int GetUserCount(this IGuild guild) =>
+            (guild as SocketGuild).MemberCount;
+
+        public static int GetCachedUserCount(this IGuild guild) =>
+            (guild as SocketGuild).DownloadedMemberCount;
+    }
+}