Fix issue with proxy server auth in chrome headless mode.
When using a proxy server that has authentication with Chrome in headless mode, SeleniumBase will now use PyVirtualDisplay (already implemented in https://github.com/seleniumbase/SeleniumBase/blob/master/seleniumbase/fixtures/base_case.py) rather than using Chrome's built-in headless mode because Chrome doesn't allow extensions when using headless mode. This assumes that the user's system already supports Xvfb (the headless display system) in order to run with PyVirtualDisplay.
See https://bugs.chromium.org/p/chromium/issues/detail?id=706008 , where a member from the Chromium team says: "Headless mode doesn't currently support extensions".