Add Support for Headless Chrome

[EndangeredMassa]

Headless Chrome is finally here!

As of Chrome 59, you can run it with:

$ chrome --headless --remote-debugging-port=9222 https://chromium.org

This is not yet a stable version of Chrome, but we could start trying it out.

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Headless Firefox is also in the works, but not nearly ready yet. It's worth keeping an eye on, though.

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Because of headless Chrome, the maintainer of the PhantomJS project is stepping down. The future of the project is uncertain. I think we can continue to support the latest version of it, but we should keep an eye on the project. If it officially ends development, we should at least warn testium users that relying on it won't be a good solution going forward.

[mykmelez]

Headless Firefox is also in the works, but not nearly ready yet. It's worth keeping an eye on, though.

Note that Firefox 55 (currently in beta) will support headless browsing on Linux, per this announcement:

https://groups.google.com/forum/#!topic/firefox-dev/TEhvuBXcJCg

Beta builds are available from https://www.mozilla.org/en-US/firefox/channel/desktop/.