code: function ( mask ) {
  
          if ( mask && ( mask instanceof RegExp ) !== true ) {
  
              throw { type: 'getURLs', message: 'First argument must be RegExp' };
  
          }
  
          return urls.map( function ( value ) {
  
              return _getURLValue( value );
  
          } ).filter( function ( value, pos, self ) {
  
              var unique = self.indexOf( value ) === pos;
  
              if ( mask && unique ) {
  
                  return mask.test( value );
  
              } else {
  
                  return unique;
  
              }
  
          } );
  
      }
docs: Returns list of unique URLs in css document.
  
  @param  {regexp} mask RegExp to test URLs against.
  @return {array}  Array of matchet URLs.