Allow multiple audit categories to be run in a single assertion

[joshmcarthur]

Running categories one per example is suboptimal, because it requires a full browser start up and Lighthouse run, just to pluck a different key from the result JSON.

It should be pretty simple for this library to accept a list of categories to run instead of a single one (since the 'score' args etc are kwargs, we can just accept multiple categories before grabbing the kwargs).

The invocation would look like it {expect("https://example.com").to pass_lighthouse_audit(:accessibility, :pwa, score: 85).

We might also want to consider an alternative syntax, with a hash of audit category name => expected score