core(runner): support multiple output modes

[patrickhulce]

returns support for multiple outputs

I know there's some ugliness in config to get this to work with unified config/CLI settings, but that'll need some good cleanup for type checking soon anyhow

biggest question for 3.0: do we want to have report/output types be string|string[] or string[]

I did the | because it's easier impl wise and won't break common output: 'html' case, but it's new functionality anyhow so now's our chance

[paulirish]

an idea:

What if the node module always returned an object.
node module also offers lighthouse.generateReport(lhr, type) method publicly.

it takes lhr as first arg. type of string 'html' 'json' or 'csv' as second arg.

but tbh i don't know how it should look in the CLI.

[brendankenny]

I haven't looked at the code yet, but agree on string|Array<string>. I think the common case will still be the single output mode, so no need to make it harder for that one

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