Add a recursiveImports option

This mimics the functionality compass introduced to allow imports on sub-directories without specifying the folder. At work we use Scout (http://mhs.github.io/scout-app/) for SASS development and recently we've integrated in gulp-ruby-sass into our build process. We found that there was a feature in compass that will look through all sub-directories for imports instead of specifying the folder yourself. I'd imagine other people would also be interested in this feature since it's automatically built into Scout/Compass.