Feature request: ability to define the screenshot path #31

Currently all of the screenshots are saved in to a single folder. This is not an ideal solution if you are running cross-browser tests on multiple things as it results in hundreds of screenshots in a single directory.

I would propose to create a nested folder structure.

screenshots/
    MyTest/
        identifier.png
        identifier-2.png

Ideally one should be able to fully define the structure. This is what my ideal structure would look like:

screenshots/
    MyTest/
        android/
            identifier.png
            identifier-2.png
        chrome/
            identifier.png
            identifier-2.png