add prachathai67k
Test failing for same issues as https://github.com/huggingface/datasets/pull/939
Please advise.

```
=========================== short test summary info ============================
FAILED tests/test_dataset_common.py::RemoteDatasetTest::test_builder_class_flue
FAILED tests/test_dataset_common.py::RemoteDatasetTest::test_builder_class_norwegian_ner
FAILED tests/test_dataset_common.py::RemoteDatasetTest::test_builder_configs_flue
FAILED tests/test_dataset_common.py::RemoteDatasetTest::test_builder_configs_norwegian_ner
FAILED tests/test_dataset_common.py::RemoteDatasetTest::test_load_dataset_flue
FAILED tests/test_dataset_common.py::RemoteDatasetTest::test_load_dataset_norwegian_ner
FAILED tests/test_dataset_common.py::RemoteDatasetTest::test_load_dataset_xglue
===== 7 failed, 1309 passed, 932 skipped, 11 warnings in 166.71s (0:02:46) =====
``` Closing and opening a new pull request to solve rebase issues To be continued on https://github.com/huggingface/datasets/pull/982
`prachathai-67k`: News Article Corpus and Multi-label Text Classificdation from Prachathai.com
The prachathai-67k dataset was scraped from the news site Prachathai.
We filtered out those articles with less than 500 characters of body text, mostly images and cartoons.
It contains 67,889 articles wtih 12 curated tags from August 24, 2004 to November 15, 2018.
The dataset was originally scraped by @lukkiddd and cleaned by @cstorm125.
You can also see preliminary exploration at https://github.com/PyThaiNLP/prachathai-67k/blob/master/exploration.ipynb