Feature request: Add an optional filename field to form

Thanks for your amazing work in advance. I'm wondering if it's possible to add an optional filename field to the form, which can override the default OUTPUT_TEMPLATE. (Optional means that if the value is not present in form, fallback to use OUTPUT_TEMPLATE.)

It's useful for downloading content with random title or metadata (e.g. downloading m3u8 content), and allows user to manage the downloaded files more intuitively without remembering the mapping between file names.