[Bug] Won't write to output folder

- [x] I didn't find a similar issue already open.
- [x] I read the documentation (README AND Wiki)
- [x] I have installed FFMpeg
- [x] My problem is related to Spleeter only, not a derivative product (such as Webapplication, or GUI provided by others)

## Description

When adding the -o command, Spleeter gives me an error and won't run properly. However, if I omit the -o command, Spleeter will run, but create the files in a temp folder

## Step to reproduce
executing `spleeter separate -o output -p spleeter:4stems /input/audio_example.mp3` won't work
executing `spleeter separate -p spleeter:4stems /input/audio_example.mp3` does work, but creates files in C:\Users\User\AppData\Local\Temp\separated_audio

## Output

```
Traceback (most recent call last):
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\runpy.py", line 192, in _run_module_as_main
return _run_code(code, main_globals, None,
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\runpy.py", line 85, in _run_code
exec(code, run_globals)
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\spleeter\__main__.py", line 262, in
entrypoint()
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\spleeter\__main__.py", line 256, in entrypoint
spleeter()
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\typer\main.py", line 214, in __call__
return get_command(self)(*args, **kwargs)
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\click\core.py", line 829, in __call__
return self.main(*args, **kwargs)
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\click\core.py", line 782, in main
rv = self.invoke(ctx)
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\click\core.py", line 1259, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\click\core.py", line 1066, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\click\core.py", line 610, in invoke
return callback(*args, **kwargs)
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\typer\main.py", line 497, in wrapper
return callback(**use_params) # type: ignore
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\spleeter\__main__.py", line 128, in separate
separator.separate_to_file(
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\spleeter\separator.py", line 383, in separate_to_file
self.save_to_file(
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\site-packages\spleeter\separator.py", line 448, in save_to_file
os.makedirs(directory)
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\os.py", line 211, in makedirs
makedirs(head, exist_ok=exist_ok)
File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\os.py", line 221, in makedirs
mkdir(name, mode)
FileNotFoundError: [WinError 2] The system cannot find the file specified: 'output'
```

## Environment

| | |
| ----------------- | ------------------------------- |
| OS | Windows |
| Installation type | pip |
| RAM available | 16GB |
| Hardware spec | N/A |