Add option to specify download directory for assets (#989)

Co-authored-by: Oleksii Holub <1935960+Tyrrrz@users.noreply.github.com>

DiscordChatExporter.Cli/Commands/Base/ExportCommandBase.cs

@@ -98,6 +98,20 @@ public abstract class ExportCommandBase : TokenCommandBase
     )]
     public bool ShouldReuseAssets { get; init; }
 
+    private readonly string? _assetsPath;
+
+    [CommandOption(
+        "media-dir",
+        Description = "Download assets to this directory."
+    )]
+    public string? AssetsPath
+    {
+        get => _assetsPath;
+        // Handle ~/ in paths on Unix systems
+        // https://github.com/Tyrrrz/DiscordChatExporter/pull/903
+        init => _assetsPath = value is not null ? Path.GetFullPath(value) : null;
+    }
+    
     [CommandOption(
         "dateformat",
         Description = "Format used when writing dates."
@@ -124,6 +138,14 @@ public abstract class ExportCommandBase : TokenCommandBase
             );
         }
 
+        // Assets directory should only be specified when the download assets option is set
+        if (!string.IsNullOrWhiteSpace(AssetsPath) && !ShouldDownloadAssets)
+        {
+            throw new CommandException(
+                "Option --media-dir cannot be used without --media."
+            );
+        }
+
         // Make sure the user does not try to export all channels into a single file.
         // Output path must either be a directory, or contain template tokens.
         // https://github.com/Tyrrrz/DiscordChatExporter/issues/799
@@ -172,6 +194,7 @@ public abstract class ExportCommandBase : TokenCommandBase
                                     guild,
                                     channel,
                                     OutputPath,
+                                    AssetsPath,
                                     ExportFormat,
                                     After,
                                     Before,