code: function(options) {
          var params = [],
              opts = options || {},
              transform = 'contrast';
  
          if (opts.sharpen) {
              params.push('sharpen=' + opts.sharpen);
          }
  
          if (params.length) {
              transform += ':' + params.join(',');
          }
  
          return this.append(transform);
      }
docs: Adjust contrast in the image
  
  @param  {Object} [options={}]
  @param  {Number} [options.sharpen] Change in contrast given as number of steps up (positive
  number) or down (negative number)
  @return {Imbo.ImageUrl}