Get the contents of HTML pages through jsdom.
@param  {Array}   files   List of HTML files
@param  {Object}  options UnCSS options
@return {Array|Promise}
function getHTML(files, options) {
    if (_.isString(files)) {
        files = [files];
    }

    files = _.flatten(files.map((file) => {
        if (!isURL(file) && !isHTML(file)) {
            return glob.sync(file);
        }
        return file;
    }));

    if (!files.length) {
        return Promise.reject(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)));
}