[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.


> 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.

wouldn't be nicer if we can have `manual_dir/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.
> 
> wouldn't be nicer if we can have `manual_dir/wikihow`?

Sure, I mean the user can decide whatever he likes best :-) The path one puts in `data_dir` will be used as the path to the manual dir. `nlp.load_dataset("wikihow", data_dir="~/manual_dir/wikihow")` would work as well as any other path ;-)   Perfect! You can merge!