Add stylus inline sprite features to CSS.  

Set locals.inlineSpriteMaxBytes to max bytes to inline.  Defaults to 0.

This exposes the functionality in Stylus described here: http://bengourley.co.uk/using-stylus

This change only takes effect if a variable is config'd in locals.json:

    {
      "locals": {
          "inlineSpriteMaxBytes" : 10000
      }
    }