async function getHTML(files, options) {
    if (_.isString(files)) {
        files = [files];
    }

    files = _.flatten(
        files.map(file => {
            if (!isURL(file) && !isHTML(file)) {
                return glob.sync(file).map(match => match.split('/').join(path.sep));
            }
            return file;
        })
    );

    if (!files.length) {
        throw new Error('UnCSS: no HTML files found');
    }

    // Save files for later reference.
    options.files = files;
    return Promise.all(files.map(file => jsdom.fromSource(file, options)));
}
Get the contents of HTML pages through jsdom.
@param  {Array}   files   List of HTML files
@param  {Object}  options UnCSS options
@return {Array|Promise}