Option to create snapshots in "light-mode" and/or "dark-mode" / prefers-color-scheme

A lot of websites have styles for light and dark mode.
`@media (prefers-color-scheme: dark) {}`
It would be useful to add a config per scenario to get screenshots in light/default, dark or both.
Puppeteer offers page.emulateMedia.

https://github.com/puppeteer/puppeteer/pull/5012
https://github.com/tomayac/dark-mode-screenshot/blob/master/dark-mode-screenshot.js#L42