added semeval18_emotion_classification dataset
I added the data set of SemEval 2018 Task 1 (Subtask 5) for emotion detection in three languages.

```
datasets-cli test datasets/semeval18_emotion_classification/ --save_infos --all_configs

RUN_SLOW=1 pytest tests/test_dataset_common.py::LocalDatasetTest::test_load_real_dataset_semeval18_emotion_classification
```
Both commands ran successfully.

I couldn't create the dummy data (the files are tsvs but have .txt ending, maybe that's the problem?) and therefore the test on the dummy data fails, maybe someone can help here.

I also formatted the code:
```
black --line-length 119 --target-version py36 datasets/semeval18_emotion_classification/
isort datasets/semeval18_emotion_classification/
flake8 datasets/semeval18_emotion_classification/
```
That's the publication for reference:

Mohammad, S., Bravo-Marquez, F., Salameh, M., & Kiritchenko, S. (2018). SemEval-2018 task 1: Affect in tweets. Proceedings of the 12th International Workshop on Semantic Evaluation, 1–17. https://doi.org/10.18653/v1/S18-1001
For training the multilabel classifier, I would combine the labels into a list, for example for the English dataset:

```
dfpre=pd.read_csv(path+"2018-E-c-En-train.txt",sep="\t")
dfpre['list'] = dfpre[dfpre.columns[2:]].values.tolist()
df = dfpre[['Tweet', 'list']].copy()
df.rename(columns={'list': 'labels'}, inplace=True)
``` Hi @maxpel , have you had a chance to take my comments into account ?

Let me know if you have questions or if I can help :) Hi @lhoestq ! I did take your comments into account, changed the naming and tried to add dummy data (manually). I am not sure if the dummy data is correct, maybe you can take a look at that.
The model card is still missing as I am currently very busy. Thanks ! The dummy data looks all good, good job :)

The CI error can be fixed by merging `master` into your branch
```bash
git fetch upstream
git merge upstream/master
``` Hi! I just added the model card and I did the merge you showed above. Should I then add and commit again? The CI error is still there right now. @lhoestq Unfortunately, I discovered a problem with the test data sets on the competion page (train and dev is fine). They still contain NONE labels for each of the emotions, for example for English: http://saifmohammad.com/WebDocs/AIT-2018/AIT2018-DATA/AIT2018-TEST-DATA/semeval2018englishtestfiles/2018-E-c-En-test.zip
Luckily, a zip file with all data of the competition contains the correct labels also for the test set:
http://saifmohammad.com/WebDocs/AIT-2018/AIT2018-DATA/SemEval2018-Task1-all-data.zip
What's the best way to correct this? Hi ! I think we can edit the sem_eval_2018_task_1.py file to use this URL instead, and maybe update the `os.path.join` calls to the new paths to the text data in the new ZIP file. Would you like to try to make this work ?