🚀 Add JSON lines output

[ocervell]

It could be cool to have a JSON lines output that prints JSON lines to the terminal, so that we can pass results in real-time to other tools. For instance an output flag -o json

Full disclosure: we found httpit so good at Freelabz that we want to integrate it into secator (https://github.com/freelabz/secator), and having this kind of output format helps a lot for integration.