Allow custom output filename format when using GUI

[bilball42]

Hi, I'd love the option to customize the output filename format in the GUI. Currently, media players that rely on exact filename matching (Heresphere) require subtitles to match the video filename exactly (e.g. myvideo.srt), but the output currently includes additional tags like myvideo.en.whisperjav.srt, requiring a rename before they're usable.

I can work around this with the free program 'Bulk Rename Utility', but a simple output format option in the GUI would be a great QoL improvement especially for users processing large batches of files.

Thanks, love this project!