Parse a given uncssrc file.
@param  {String} filename The location of the uncssrc file
@return {Object}          The options object
function parseUncssrc(filename) {
    let options = JSON.parse(fs.readFileSync(filename, 'utf-8'));

    /* RegExps can't be stored as JSON, therefore we need to parse them manually.
     * A string is a RegExp if it starts with '/', since that wouldn't be a valid CSS selector.
     */
    options.ignore = options.ignore ? options.ignore.map(strToRegExp) : undefined;
    options.ignoreSheets = options.ignoreSheets ? options.ignoreSheets.map(strToRegExp) : undefined;

    return options;
}