Added the image_path option to the middleware.

lib/middleware.js

@@ -120,6 +120,7 @@ module.exports = function(options){
             });
           }, {
             include_paths: [ sassDir ].concat(options.include_paths || options.includePaths || []),
+            image_path: options.image_path || options.imagePath,
             output_style: options.output_style || options.outputStyle
           });
         });