Function throttle

docs/javascripts/readingProgress.js:126–135

(func, limit)

    // 节流函数
    function throttle(func, limit) {
        let inThrottle;
        return function() {
            if (!inThrottle) {
                func.apply(this, arguments);
                inThrottle = true;
                setTimeout(() => inThrottle = false, limit);
            }
        }
    }

    // 初始化
    function init() {