cli: fix --help/readme to say output paths for multiple report types is supported

[connorjclark]

Ran into this limitation when I was a LH user. There is currently no way to specify the output path if you wish to receive multiple report formats.

Suggested interface:

lighthouse https://www.cnn.com --output json,html --output-path out/my-lhr.json,out/my-other-lhr.csv

[brendankenny]

you can specify a path for all of them, just not individual paths. If calling on the CLI, it seems trivial to do --output-path path/my-lhr and get path/my-lhr.report.json, path/my-lhr.report.csv, path/my-lhr.report.html, then move them if you really want a completely custom name.

[connorjclark]

That would have covered my use case just fine. In fact, that's probably what I did, I'm just confused on the matter now. Could just update the readme then, since there's this:

If using multiple outputs, --output-path is ignored.

I think I just recall being confused by that.

[brendankenny]

oh, ha, yeah :( that's hilariously out of date