browserOverride should accept a browser instance instead of a brittle config object

[bartveneman]

ATM, browserOverride is a complex piece of if-statements and a combination of fields, but actually it should accept a fully configured browser instance.

Example:

const browser = puppeteer.launch(browserOptions)
const css = extractCss(url, {browserOverride: browser})

We should build in some sanity checks that the browser has at least the correct interface in regard to the following methods:

• newPage()
• close()