[Manual downloads] add logic proposal for manual downloads and add wikihow
Wikihow is an example that needs to manually download two files as stated in: https://github.com/mahnazkoupaee/WikiHow-Dataset. 

The user can then store these files under a hard-coded name: `wikihowAll.csv` and `wikihowSep.csv` in this case in a directory of his choice, e.g. `~/wikihow/manual_dir`.

The dataset can then be loaded via:

```python
import nlp
nlp.load_dataset("wikihow", data_dir="~/wikihow/manual_dir")
```

I added/changed so that there are explicit error messages when using manually downloaded files.

