**************************************************************** 7. 输出修改后的样式表 **************************************************************** 
输出修改后的样式表
@param  {SpriteTask} spriteTask
function exportCssFile(spriteTask) {
    var cssContentList = [],
        styleSheetArray = spriteTask.styleSheetArray,
        cssContent = '',
        compressOptions = spriteConfig.output.compress,
        fileName,
        fileName2, // 用于输出 log
        combinedCssRules,
        imageName,
        fileversion = spriteTask.fileversion;

    if (!styleSheetArray) {
        styleSheetArray = [spriteTask.styleSheet]
    }

    styleSheetArray.forEach(function(styleSheet) {
        cssContentList.push(styleSheetToString(styleSheet));
    });

    fileName = path.basename(spriteTask.cssFileName);

    fileName2 = path.join(spriteConfig.output.cssDist, fileName);
    fileName2 = path.normalize(fileName2);

    fileName = path.join(spriteConfig.workspace, fileName2);
    fileName = path.resolve(fileName);

    // 把合并了的样式统一在一起输出到文件的最后
    if (spriteConfig.output.combineCSSRule) {
        combinedCssRules = spriteTask.combinedCssRules;
        for (imageName in combinedCssRules) {
            var versionImageName = imageName;
            if (fileversion) {
                versionImageName += '?' + fileversion;
            }
            cssContent += combinedCssRules[imageName].join(',') + '{' +
                'background-image: url(' + versionImageName + ');' +
                '}\n';
        }
    }

    cssContent = cssContentList.join('\n') + cssContent;
    if (compressOptions) { // 压缩
        if (!us.isObject(compressOptions)) {
            compressOptions = null;
        }
        cssContent = new CleanCSS(compressOptions).minify(cssContent);
    }
    nf.writeFileSync(fileName, cssContent, true);
    info('>>Output css:', fileName2);
}