Adding `sep` to TextConfig 
I have a local a `.txt` file that follows the `CONLL2003` format which I need to load using `load_script`. However, by using `sample_by='line'`, one can only split the dataset into lines without splitting each line into columns. Would it be reasonable to add a `sep` argument in combination with `sample_by='paragraph'` to parse a paragraph into an array for each column ? If so, I am happy to contribute!

## Environment
* `python 3.8.10`
* `datasets 2.9.0`

## Snippet of `train.txt`

```txt
Distribution	NN	O	O
and	NN	O	O
dynamics	NN	O	O
of	NN	O	O
electron	NN	O	B-RP
complexes	NN	O	I-RP
in	NN	O	O
cyanobacterial	NN	O	B-R
membranes	NN	O	I-R

The	NN	O	O
occurrence	NN	O	O
of	NN	O	O
prostaglandin	NN	O	B-R
F2α	NN	O	I-R
in	NN	O	O
Pharbitis	NN	O	B-R
seedlings	NN	O	I-R
grown	NN	O	O
under	NN	O	O
short	NN	O	B-P
days	NN	O	I-P
or	NN	O	I-P
days	NN	O	I-P

```

## Current Behaviour 
```python
# defining 4 features ['tokens', 'pos_tags', 'chunk_tags', 'ner_tags'] here would fail with `ValueError: Length of names (4) does not match length of arrays (1)`
dataset = datasets.load_dataset(path='text', features=features, data_files={'train': 'train.txt'}, sample_by='line')
dataset['train']['tokens'][0]
>>> 'Distribution\tNN\tO\tO'
```

## Expected Behaviour / Suggestion
```python
# suppose we defined 4 features ['tokens', 'pos_tags', 'chunk_tags', 'ner_tags']
dataset = datasets.load_dataset(path='text', features=features, data_files={'train': 'train.txt'}, sample_by='paragraph', sep='\t')
dataset['train']['tokens'][0]
>>> ['Distribution', 'and', 'dynamics', ... ]

dataset['train']['ner_tags'][0]
>>> ['O', 'O', 'O', ... ]
```
Hi @omar-araboghli, thanks for your proposal.

Have you tried to use "csv" loader instead of "text"? That already has a `sep` argument. Hi @albertvillanova, thanks for the quick response!

Indeed, I have been trying to use `csv` instead of `text`. However I am still not able to define range of rows as one sequence, that is achievable with passing `sample_by='paragraph'` to the `TextConfig`

For instance, the below code

```python
import datasets

dataset = datasets.load_dataset(
    path='csv',
    data_files={'train': TRAINING_SET_PATH},
    sep='\t',
    header=None,
    column_names=['tokens', 'pos_tags', 'chunk_tags', 'ner_tags']
)
```

leads to 

```python
dataset
>>> DatasetDict({
    train: Dataset({
        features: ['tokens', 'pos_tags', 'chunk_tags', 'ner_tags'],
        num_rows: 62543
    })
})

dataset['train'][0]
>>> {'tokens': 'Distribution',
 'pos_tags': 'NN',
 'chunk_tags': 'O',
 'ner_tags': 'O'
}
```
Is there a way to deal with multiple csv rows as one dataset instance, where each column is a sequence of those rows ?