[CLI] Add command to export all channels across all guilds (#373)

DiscordChatExporter.Cli/Commands/ExportAllCommand.cs

@@ -0,0 +1,33 @@
+using System.Linq;
+using System.Threading.Tasks;
+using CliFx;
+using CliFx.Attributes;
+using DiscordChatExporter.Cli.Commands.Base;
+using DiscordChatExporter.Domain.Discord.Models;
+using DiscordChatExporter.Domain.Utilities;
+
+namespace DiscordChatExporter.Cli.Commands
+{
+    [Command("exportall", Description = "Export all direct messages and all channels within all guilds.")]
+    public class ExportAllCommand : ExportMultipleCommandBase
+    {
+        [CommandOption("exclude-dm", 'e', Description = "If this flag is present, direct messages will not be exported.")]
+        public bool ExcludeDMs { get; set; }
+        
+        public override async ValueTask ExecuteAsync(IConsole console)
+        {
+
+            if(!ExcludeDMs){
+                var dmChannels = await GetDiscordClient().GetGuildChannelsAsync(Guild.DirectMessages.Id);
+                await ExportMultipleAsync(console, dmChannels);
+            }
+
+            var guilds = await GetDiscordClient().GetUserGuildsAsync();
+            foreach (var guild in guilds.OrderBy(g => g.Name))
+            {
+                var guildChannels = await GetDiscordClient().GetGuildChannelsAsync(guild.Id);
+                await ExportMultipleAsync(console, guildChannels);
+            }
+        }
+    }
+}