Add option to run headless

[RichiCoder1]

Both firefox and chrome (with some noise from Edge) support headless, and the selenium images referenced in the readme allow disabling running the display server if the inner browsers support headless. Would be great to either A) have a headless option or B) have the abilities to append capabilities.

[L0wry]

Hey @RichiCoder1, personally we're not a fan of headless browsers as we have had bad experiences in the past where they have introduced flake or allowed bugs through.
If you think about headless browsers as a concept they are a step further away from how your end user interacts with the page and as such introduces some level of risk.

However, adding support for headless would be super easy, I'll get something on the backlog.

[RichiCoder1]

I may look into adding this then. I agree with the risks of headless for automation testing, but since this is purely rendering and (for most cases) doesn't include interaction, I think this is something headless could be good for.