fix highlighting on posts and pages

src/Article.js

@@ -1,3 +1,4 @@
+import hljs from 'highlight.js';
 import React, { Component } from 'react';
 import Icon from 'react-fontawesome';
 import Markdown from 'react-markdown';
@@ -34,6 +35,12 @@ class Article extends Component {
     );
   }
 
+  componentDidMount() {
+    document.querySelectorAll('pre code').forEach(elm => {
+      hljs.highlightBlock(elm);
+    });
+  }
+
   postLink() {
     let date = new Date(this.props.article.date);
     let year = date.getUTCFullYear();