Added OPUS ParaCrawl
@lhoestq I saw some common changes you made on the other PR's (Similar Opus Datasets). I fixed those changes here. Can you please review it once ? 
Thanks. @rkc007 @lhoestq just noticed a dataset named para_crawl has been added a long time ago: #91. They're not exactly the same so it's ok to have both.

Especially the `para_crawl` that already exists only uses the text from the ParaCrawl release 4. Could you regenerate the dataset_infos.json @rkc007 please ?
It looks like it has some issues due to the dataset class name change @SBrandeis Thank you for suggesting changes. I made the changes you suggested. 

@lhoestq I generated `dataset_infos.json` again. I ran both tests(Dummy & Real data) and it passed. Can you please review it again? merging since the CI is fixed on master
Dataset : http://opus.nlpl.eu/ParaCrawl.php