Expose the ability to take screenshots

I understand Karma is primarily designed for unit tests, however the ability to screenshot during a test would be very useful for visual regression testing (both automated and manual).

End-to-end frameworks like Protractor or raw WebDriver scripts generally require you handle building the frontend and spinning up a server. Karma already does almost exactly what I need for this use case, except the ability to take a screenshot.

It's currently possible [via the PhantomJS launcher](https://stackoverflow.com/questions/34694765/take-screenshot-from-karma-while-running-tests-in-phantomjs-2), but it would be great if more launchers supported it, especially in a cross-browser way.