Make css output easier to debug

[alextea]

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

Example source comments:

/* line 18, govuk_modules/govuk_frontend_toolkit/stylesheets/_grid_layout.scss */
#content {
  max-width: 960px;
  margin: 0 15px;
}

@media (min-width: 641px) {
  /* line 18, govuk_modules/govuk_frontend_toolkit/stylesheets/_grid_layout.scss */
  #content {
    margin: 0 30px;
  }
}

[gemmaleigh]

Nice!

Works for me, here's a couple of screenshots.

on master (without this change):

on this branch:

I've rebased on top of master, happy for you to merge when the tests are complete @alextea.

[gemmaleigh]

I've tested this locally, it works well - I've added screenshots to the comments to show it has been tested.