Add support for different headless browsers

[waarissyb]

After public signals about development of a headless Chrome browser, the maintainer of PhantomJS announced he’s stepping down. There’s also development for a headless mode in Firefox.

I’ve experienced quite some JS issues with PhantomJS. Visual regression testing is even more necessary in interfaces that get enhanced by JS. That’s because changes in CSS are more predictable to cause regression. With developments for headless Chrome and Firefox, the latest JS features become available for Argus Eyes.

Currently headless Chrome is available for MacOS and Linux, whereas headless Firefox is only available on Linux.