function bytediffFormatter(data) {
    var difference = (data.savings > 0) ? ' smaller.' : ' larger.';
    return data.fileName + ' went from ' +
        (data.startSize / 1000).toFixed(2) + ' kB to ' + (data.endSize / 1000).toFixed(2) + ' kB' +
        ' and is ' + formatPercent(1 - data.percent, 2) + '%' + difference;
}
Formatter for bytediff to display the size changes after processing
@param  {Object} data - byte data
@return {String}      Difference in bytes, formatted