input: Write a function named `parse_arguments` that parses the command-line arguments for the Subtitles Client script. The function should accept a list of arguments and return a dictionary with keys corresponding to the available commands ("download", "exist", "test", "list", "help", "version") and their respective values. If the command is not recognized, it should default to showing the help screen.
  
  assert parse_arguments(["download", "movie.mkv", "--lang=en", "--engine=opensubtitles"])=={"command": "download", "releases_path": ["movie.mkv"], "language": "en", "engine": ["opensubtitles"]}
  assert parse_arguments(["exist", "tv_show.mkv", "episode2.mkv"])=={"command": "exist", "releases_path": ["tv_show.mkv", "episode2.mkv"], "language": "en", "engine": ["opensubtitles"]}
  assert parse_arguments(["test", "opensubtitles"])=={"command": "test", "engines": ["opensubtitles"]}
  assert parse_arguments(["list"])=={"command": "list"}
  assert parse_arguments(["-h"])=={"command": "help"}
  assert parse_arguments(["-v"])=={"command": "version"}
  assert parse_arguments(["unknown"])==None
output: def parse_arguments(args):
      commands = {"download", "exist", "test", "list", "help", "version"}
      if not args or args[0] not in commands:
          return None
      
      result = {"command": args[0]}
      
      if args[0] == "download":
          result["releases_path"] = [args[1]]
          result["language"] = "en"
          result["engine"] = []
          for arg in args[2:]:
              if arg.startswith("--lang="):
                  result["language"] = arg.split("=")[1]
              elif arg.startswith("--engine="):
                  result["engine"].append(arg.split("=")[1])
      elif args[0] == "exist":
          result["releases_path"] = args[1:]
          result["language"] = "en"
          result["engine"] = ["opensubtitles"]
      elif args[0] == "test":
          result["engines"] = args[1:]
      elif args[0] in ["help", "version"]:
          pass
      elif args[0] == "list":
          pass
      
      return result