Allow for PathLike objects
Using PathLike objects as input for `load_dataset` does not seem to work. The following will throw an error.

```python
files = list(Path(r"D:\corpora\yourcorpus").glob("*.txt"))
dataset = load_dataset("text", data_files=files)
```

Traceback:

```
Traceback (most recent call last):
  File "C:/dev/python/dutch-simplification/main.py", line 7, in <module>
    dataset = load_dataset("text", data_files=files)
  File "C:\Users\bramv\.virtualenvs\dutch-simplification-nbNdqK9u\lib\site-packages\nlp\load.py", line 548, in load_dataset
    builder_instance.download_and_prepare(
  File "C:\Users\bramv\.virtualenvs\dutch-simplification-nbNdqK9u\lib\site-packages\nlp\builder.py", line 470, in download_and_prepare
    self._save_info()
  File "C:\Users\bramv\.virtualenvs\dutch-simplification-nbNdqK9u\lib\site-packages\nlp\builder.py", line 564, in _save_info
    self.info.write_to_directory(self._cache_dir)
  File "C:\Users\bramv\.virtualenvs\dutch-simplification-nbNdqK9u\lib\site-packages\nlp\info.py", line 149, in write_to_directory
    self._dump_info(f)
  File "C:\Users\bramv\.virtualenvs\dutch-simplification-nbNdqK9u\lib\site-packages\nlp\info.py", line 156, in _dump_info
    file.write(json.dumps(asdict(self)).encode("utf-8"))
  File "c:\users\bramv\appdata\local\programs\python\python38\lib\json\__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "c:\users\bramv\appdata\local\programs\python\python38\lib\json\encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "c:\users\bramv\appdata\local\programs\python\python38\lib\json\encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
TypeError: keys must be str, int, float, bool or None, not WindowsPath
```

We have to cast to a string explicitly to make this work. It would be nicer if we could actually use PathLike objects.

```python
files = [str(f) for f in Path(r"D:\corpora\wablieft").glob("*.txt")]
```
