Machine readable output option

[seagreen]

I may have missed it, but if not it would be good to have some kind of machine readable output format (JSON / TSV / etc).

[seagreen]

If you think this is a good idea I can make a PR for it.

[knupfer]

I think it would be a great idea and I'd love to get some contributions. Go for it!

[seagreen]

Good news: we already have aeson as a transitive dependency because of chronos, so it will be easy to output JSON.

I do have a question about how we want things to work.

Two options:

1. A --json flag that acts like --print-once but prints as JSON instead of the current output bars.

2. We switch the current output to be done over STDERR instead of STDOUT. This would be a big change, but makes sense (maybe?) because it's for human consumption, and can't be piped into other tools.

   Then we add a --json flag that prints results as JSON on STDOUT at the end of a run.

   This would be nice because when you use --json you can still watch the output bars to see how the benchmark is progressing like you can right now, and then ctrl+C when you're ready to get a JSON report.

What are your thoughts? I prefer the second, but that may just be because I haven't realized some downside to it yet, it would certainly be a big change.

[seagreen]

After talking with a friend about it, I changed my mind and like the simplicity of the current way of doing things, where everything is emitted on STDOUT.

That means we'll probably want the --json flag to prevent showing of the charts.

I've made a tentative PR for us to look at here: #6