refactor(core): adjust directory structure:

src/core/fetch/index.js

@@ -0,0 +1,13 @@
+import { callHook } from '../init/lifecycle'
+
+export function fetchMixin (Docsify) {
+  Docsify.prototype.$fetch = function () {
+  }
+}
+
+export function initFetch (vm) {
+  vm.$fetch(result => {
+    vm.$resetEvents()
+    callHook(vm, 'doneEach')
+  })
+}