feat: add optional filename_format parameter to download_videos()

[SamStorer]

• Add _format_filename_default() method to encapsulate default filename formatting
• Add filename_format parameter to download_videos() allowing custom filename generation
• Update _parse_downloaded_items() to accept and use custom format function
• Maintain full backward compatibility (defaults to existing slugified format)
• Add comprehensive documentation and examples
• Add test cases for custom format functions

Usage example:

    def custom_fmt(created_at, camera_name, path):
        dt = datetime.datetime.fromisoformat(created_at).astimezone(
            pytz.timezone('US/Eastern')
        )
        clean_name = camera_name.replace(' ', '')
        return os.path.join(path, f'{dt:%Y%m%d_%H%M%S}_{clean_name}.mp4')

    await blink.download_videos(path, filename_format=custom_fmt)