Make css output easier to debug

The generated css will now include source comments which show where in 
your sass files each css block comes from.

Also tidied up the options block:

 - The `style` option is not supported by grunt-sass (`outputStyle` is the 
correct option)
- `sourceMap` was misspelt and wasn’t generating the source maps
- The options have been arranged in alphabetical order to make it easier 
to read

Gruntfile.js

@@ -4,14 +4,14 @@ module.exports = function (grunt) {
     sass: {
       dev: {
         options: {
-          style: 'expanded',
-          sourcemap: true,
           includePaths: [
             'govuk_modules/govuk_template/assets/stylesheets',
             'govuk_modules/govuk_frontend_toolkit/stylesheets',
             'govuk_modules/govuk-elements-sass/'
           ],
-          outputStyle: 'expanded'
+          outputStyle: 'expanded',
+          sourceComments: true,
+          sourceMap: true
         },
         files: [{
           expand: true,