Issue 6

This change addresses https://github.com/Wikia/vignette-js/issues/6
I'm confused on how `mode` gets set here - is it set because the legacy url defines the offsets, so the image must be one of the window-crop modes, or is it something the user of this library wants to use?

Also seems off that this is performing a regex match on `urlParameters.imagePath`, when `urlParameters.imagePath` is already part of `url` (defined just above the mode concatenations)