refactor(core): adjust directory structure:

src/core/event/index.js

@@ -0,0 +1,11 @@
+export function eventMixin (Docsify) {
+  Docsify.prototype.$bindEvents = function () {
+  }
+
+  Docsify.prototype.$resetEvents = function () {
+  }
+}
+
+export function initEvent (vm) {
+  vm.$bindEvents()
+}