customize sidebar and navbar via markdown file, #6 (#8)

* Refactor: use iife

* Feat: customize sidebar and navbar via markdown file, #6

* Add changelog

src/util.js

@@ -0,0 +1,68 @@
+/**
+ * Simple ajax
+ * @param  {String} url
+ * @param  {String} [method=get]
+ * @return {Promise}
+ */
+export function load (url, method = 'get') {
+  const xhr = new XMLHttpRequest()
+
+  xhr.open(method, url)
+  xhr.send()
+
+  return {
+    then: function (success, error = function () {}) {
+      xhr.addEventListener('error', error)
+      xhr.addEventListener('load', ({ target }) => {
+        target.status >= 400 ? error(target) : success(target.response)
+      })
+    }
+  }
+}
+
+/**
+ * gen toc tree
+ * @link https://github.com/killercup/grock/blob/5280ae63e16c5739e9233d9009bc235ed7d79a50/styles/solarized/assets/js/behavior.coffee#L54-L81
+ * @param  {Array} toc
+ * @param  {Number} maxLevel
+ * @return {Array}
+ */
+export function genTree (toc, maxLevel) {
+  const headlines = []
+  const last = {}
+
+  toc.forEach(headline => {
+    const level = headline.level || 1
+    const len = level - 1
+
+    if (level > maxLevel) return
+    if (last[len]) {
+      last[len].children = last[len].children || []
+      last[len].children.push(headline)
+    } else {
+      headlines.push(headline)
+    }
+    last[level] = headline
+  })
+
+  return headlines
+}
+
+/**
+ * camel to kebab
+ * @link https://github.com/bokuweb/kebab2camel/blob/master/index.js
+ * @param  {String} str
+ * @return {String}
+ */
+export function camel2kebab (str) {
+  return str.replace(/([A-Z])/g, m => '-' + m.toLowerCase())
+}
+
+/**
+ * is nil
+ * @param  {Object}  object
+ * @return {Boolean}
+ */
+export function isNil (o) {
+  return o === null || o === undefined
+}