nlp downloads to its module path
> Feel free to play with this change from the master branch for now, and let us know if it sounds good for you :)
> We plan to do a release in the next coming days

Thanks for making this change! I just packaged the latest commit on master and it works like a charm now! :partying_face: 
I am trying to package `nlp` for Nix, because it is now an optional dependency for `transformers`. The problem that I encounter is that the `nlp` library downloads to the module path, which is typically not writable in most package management systems:

```>>> import nlp
>>> squad_dataset = nlp.load_dataset('squad')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nix/store/2yhik0hhqayksmkkfb0ylqp8cf5wa5wp-python3-3.8.5-env/lib/python3.8/site-packages/nlp/load.py", line 530, in load_dataset
    module_path, hash = prepare_module(path, download_config=download_config, dataset=True)
  File "/nix/store/2yhik0hhqayksmkkfb0ylqp8cf5wa5wp-python3-3.8.5-env/lib/python3.8/site-packages/nlp/load.py", line 329, in prepare_module
    os.makedirs(main_folder_path, exist_ok=True)
  File "/nix/store/685kq8pyhrvajah1hdsfn4q7gm3j4yd4-python3-3.8.5/lib/python3.8/os.py", line 223, in makedirs
    mkdir(name, mode)
OSError: [Errno 30] Read-only file system: '/nix/store/2yhik0hhqayksmkkfb0ylqp8cf5wa5wp-python3-3.8.5-env/lib/python3.8/site-packages/nlp/datasets/squad'
```

Do you have any suggested workaround for this issue?

Perhaps overriding the default value for `force_local_path` of `prepare_module`?
nlp downloads to its module path 
 I am trying to package `nlp` for Nix, because it is now an optional dependency for `transformers`. The problem that I encounter is that the `nlp` library downloads to the module path, which is typically not writable in most package management systems:

```>>> import nlp
>>> squad_dataset = nlp.load_dataset('squad')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nix/store/2yhik0hhqayksmkkfb0ylqp8cf5wa5wp-python3-3.8.5-env/lib/python3.8/site-packages/nlp/load.py", line 530, in load_dataset
    module_path, hash = prepare_module(path, download_config=download_config, dataset=True)
  File "/nix/store/2yhik0hhqayksmkkfb0ylqp8cf5wa5wp-python3-3.8.5-env/lib/python3.8/site-packages/nlp/load.py", line 329, in prepare_module
    os.makedirs(main_folder_path, exist_ok=True)
  File "/nix/store/685kq8pyhrvajah1hdsfn4q7gm3j4yd4-python3-3.8.5/lib/python3.8/os.py", line 223, in makedirs
    mkdir(name, mode)
OSError: [Errno 30] Read-only file system: '/nix/store/2yhik0hhqayksmkkfb0ylqp8cf5wa5wp-python3-3.8.5-env/lib/python3.8/site-packages/nlp/datasets/squad'
```

Do you have any suggested workaround for this issue?

Perhaps overriding the default value for `force_local_path` of `prepare_module`? 
 > Feel free to play with this change from the master branch for now, and let us know if it sounds good for you :)
> We plan to do a release in the next coming days

Thanks for making this change! I just packaged the latest commit on master and it works like a charm now! :partying_face: 