Set to_json default to JSON lines
This is perfect, @albertvillanova - thank you! Tested it to work.

Might it be a good idea to document the args to `to_json`?

and also even a very basic progress bar? took 10min for 8M large records for `openwebtext` so perhaps some indication of it's being alive every min or so? @lhoestq I added tests for both `lines` and `orient`.
With this PR, the method `Dataset.to_json`:
- is added to the docs
- defaults to JSON lines