add xvfb_display

[jffifa]

Add "xvfb_display" option to "SELENIUM" field of config that allows user to run webdriver headless.

[NikolaiT]

Huge thanks!

[jffifa]

I'm afraid that you may need to merge another pull request.
As you may find, I've added another search engine "googleimg", which uses advanced google image search for my works, in my "master" branch. It added features which allow selenium to change search params for search engines which have "advanced mode".
Since it is not well tested, it may cause unpredictable bugs.
I'm sorry about my fault. I've create a new branch and revert the code ONLY containing this feature mentioned above. You may choose to merge the request on just leave it. It depends on you.