Get the contents of CSS files.
@param  {Array}   files   List of HTML files
@param  {Object}  options UnCSS options
@param  {Array}   pages   Pages opened by jsdom
@return {Promise}
function getStylesheets(files, options, pages) {
    if (options.stylesheets && options.stylesheets.length) {
        /* Simulate the behavior below */
        return Promise.resolve([files, options, pages, [options.stylesheets]]);
    }
    /* Extract the stylesheets from the HTML */
    return Promise.all(pages.map((page) => jsdom.getStylesheets(page, options)))
    .then((stylesheets) => [files, options, pages, stylesheets]);
}