refactor(core): adjust directory structure:

src/core/index.js

@@ -0,0 +1,30 @@
+import { initMixin } from './init'
+import { routeMixin } from './route'
+import { renderMixin } from './render'
+import { fetchMixin } from './fetch'
+import { eventMixin } from './event'
+import * as util from './util'
+import { get as load } from './fetch/ajax'
+import * as routeUtil from './route/util'
+
+function Docsify () {
+  this._init()
+}
+
+initMixin(Docsify)
+routeMixin(Docsify)
+renderMixin(Docsify)
+fetchMixin(Docsify)
+eventMixin(Docsify)
+
+/**
+ * Global API
+ */
+window.Docsify = {
+  util: util.merge({ load }, util, routeUtil)
+}
+
+/**
+ * Run Docsify
+ */
+setTimeout(() => new Docsify(), 0)