Add support for announcement channels

Closes #222

DiscordChatExporter.Core.Models/ChannelType.cs

@@ -1,6 +1,7 @@
 namespace DiscordChatExporter.Core.Models
 {
     // https://discordapp.com/developers/docs/resources/channel#channel-object-channel-types
+    // Order of enum fields needs to match the order in the docs.
 
     public enum ChannelType
     {
@@ -8,6 +9,8 @@
         DirectTextChat,
         GuildVoiceChat,
         DirectGroupTextChat,
-        Category
+        GuildCategory,
+        GuildNews,
+        GuildStore
     }
 }

DiscordChatExporter.Core.Models/Extensions.cs

@@ -4,6 +4,13 @@ namespace DiscordChatExporter.Core.Models
 {
     public static class Extensions
     {
+        public static bool IsExportable(this ChannelType channelType) =>
+            channelType == ChannelType.GuildTextChat ||
+            channelType == ChannelType.DirectTextChat ||
+            channelType == ChannelType.DirectGroupTextChat ||
+            channelType == ChannelType.GuildNews ||
+            channelType == ChannelType.GuildStore;
+
         public static string GetFileExtension(this ExportFormat format) =>
             format switch
             {