add web_questions
What does the `nlp-cli dummy_data` command returns ? `test.json` -> `test` 
and 
`train.json` -> `train`

as shown by the `nlp-cli dummy_data` command ;-) LGTM for merge @lhoestq - I let you merge if you want to.
add Web Question dataset
#336 

Maybe @patrickvonplaten you can help with the dummy_data structure? it still broken