filled background

@drsnyder 
Adding support for the ?fill=<color> query parameter, which supports [ImageMagick colors](http://www.imagemagick.org/script/color.php), including "transparent", which will convert the image to png format.

Note that this was branched from https://github.com/Wikia/vignette/pull/11, since that also touched route code and I wanted to avoid conflicts. 
Should this be called something like `render-to-type` or `change-render-type`? If we did that then we could add the type (e.g. "png") as a parameter. What do you think?