Viewport Screenshots for Default Workflow

[tf]

Hi,

first let me say, that this is an great library and which let me get to really good results really quickly.

While testing a website built with absolute positions, I ran into the issue described in #141. Looking through the code, I found that there already was a way to make viewport screenshots rather than document screenshots, which was exactly what I wanted. Still, the default workflow relies on makeScreenshot which only supports saveDocumentScreenshot.

So I am wondering if I should prepare a PR adding an option for the webdrivercss call, which makes the default workflow use saveViewportScreenshot. Or is there already another way or even a reason not to do it at all?

Best,
Tim