Add StackOverflow StackSample dataset

This PR adds the StackOverflow StackSample dataset from Kaggle: https://www.kaggle.com/stackoverflow/stacksample

Ran through all of the steps. However, since my dataset requires manually downloading the data, I was unable to run the pytest on the real dataset (the dummy data pytest passed).

Comments:
@lhoestq Thanks for the review and suggestions! I've added your comments and pushed the changes. I'm having issues with the dummy data still. When I run the dummy data test

```bash
RUN_SLOW=1 pytest tests/test_dataset_common.py::LocalDatasetTest::test_load_dataset_all_configs_so_stacksample
```
I get this error: 

```
___________________________________________ LocalDatasetTest.test_load_dataset_all_configs_so_stacksample ____________________________________________

self = <tests.test_dataset_common.LocalDatasetTest testMethod=test_load_dataset_all_configs_so_stacksample>, dataset_name = 'so_stacksample'

    @slow
    def test_load_dataset_all_configs(self, dataset_name):
        configs = self.dataset_tester.load_all_configs(dataset_name, is_local=True)
>       self.dataset_tester.check_load_dataset(dataset_name, configs, is_local=True)

tests/test_dataset_common.py:237: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/test_dataset_common.py:198: in check_load_dataset
    self.parent.assertTrue(len(dataset[split]) > 0)
E   AssertionError: False is not true

FAILED tests/test_dataset_common.py::LocalDatasetTest::test_load_dataset_all_configs_so_stacksample - AssertionError: False is not true
```

I tried formatting the data similar to other datasets, but I think I don't have my csv's in the zip folder with the proper name. I also ran the command that's supposed to outline the exact steps I need to perform to get them into the correct format, but I followed them and they don't seem to be working still :/. Any help would be greatly appreciated!


Ok I found the issue with the dummy data.
It's currently failing because it's not generating a single example using the dummy csv file.
That's because there's only only line in the dummy csv file, and this line is skipped using the `next()` call used to ignore the headers of the csv.

To fix the dummy data you must add headers to the dummy csv files.

Also can you make sure that all the original CSV files have headers ? i.e. check that their first line is just the column names

> Ok I found the issue with the dummy data.
> It's currently failing because it's not generating a single example using the dummy csv file.
> That's because there's only only line in the dummy csv file, and this line is skipped using the `next()` call used to ignore the headers of the csv.
> 
> To fix the dummy data you must add headers to the dummy csv files.

Oh man, I bamboozled myself! Thank you @lhoestq for catching that! I've updated the dummy csv's to include headers and also confirmed that they all have headers, so I am not throwing away any information with that `next()` call. When I run the test locally for the dummy data it passes, so hopefully it is good to go :D

merging since the Ci is fixed on master