Update asset export logic

- Disabled saving files with a unique id if they already exist. A unique id will only be added to files with identical names during export.
- Added an option to overwrite exisisting files.
- Fixed support of multiple model export using "Export selected objects (split)" option. (#43)

AssetStudioCLI/Studio.cs

@@ -761,6 +761,7 @@ namespace AssetStudioCLI
                 }
                 Console.Write($"Exported [{exportedCount}/{toExportCount}]\r");
             }
+            Exporter.ClearHash();
 
             Parallel.ForEach(toParallelExportAssetDict, new ParallelOptions { MaxDegreeOfParallelism = parallelExportCount }, toExportAsset =>
             {