feat(emoji): add emoji plugin

docs/write-a-plugin.md

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+# Write a plugin
+
+A plugin is simply a function that takes `hook` as arguments.
+The hook supports handling asynchronous tasks.
+
+## Full configuration
+
+```js
+window.$docsify = {
+ plugins: [
+  function (hook, vm) {
+    hook.init(function() {
+      // Called when the script starts running, only trigger once, no arguments,
+    })
+
+    hook.beforeEach(function(content) {
+      // Invoked each time before parsing the Markdown file.
+      // ...
+      return content
+    })
+
+    hook.afterEach(function(html, next) {
+      // Invoked each time after the Markdown file is parsed.
+      // beforeEach and afterEach support asynchronous。
+      // ...
+      // call `next(html)` when task is done.
+      next(html)
+    })
+
+    hook.doneEach(function() {
+      // Invoked each time after the data is fully loaded, no arguments,
+      // ...
+    })
+
+    hook.mounted(function() {
+      // Called after initial completion. Only trigger once, no arguments.
+    })
+
+    hook.ready(function() {
+      // Called after initial completion, no arguments.
+    })
+  }
+ ]
+}
+```
+
+!> You can get internal methods through `window.Docsify`. Get the current instance through the second argument.
+
+## Example
+
+Add footer component in each pages.
+
+```js
+window.$docsify = {
+  plugins: [
+    function (hook) {
+      var footer = [
+        '<hr/>',
+        '<footer>',
+        '<span><a href="https://github.com/QingWei-Li">cinwell</a> &copy;2017.</span>',
+        '<span>Proudly published with <a href="https://github.com/QingWei-Li/docsify" target="_blank">docsify</a>.</span>',
+        '</footer>'
+      ].join('')
+
+      hook.afterEach(function (html) {
+        return html + footer
+      })
+    }
+  ]
+}
+```