Extract stylesheets' hrefs from dom
@param  {Object}  window     A jsdom window
@param  {Object}  options    Options, as passed to UnCSS
@return {Array}
function getStylesheets(window, options) {
    if (Array.isArray(options.media) === false) {
        options.media = [options.media];
    }

    const media = _.union(['', 'all', 'screen'], options.media);
    const elements = window.document.querySelectorAll('link[rel="stylesheet"]');

    return Array.prototype.map.call(elements, (link) => ({
        href: link.getAttribute('href'),
        media: link.getAttribute('media') || ''
    }))
    .filter((sheet) => media.indexOf(sheet.media) !== -1)
    .map((sheet) => sheet.href);
}