added Markdown support to Post.js

Post.js.diff

-import React from 'react';
+import React, { createElement } from 'react';
 import { createClient } from 'contentful';
 import Helmet from 'react-helmet';
+import marksy from 'marksy';
+
+// Markdown helper function
+const getMarkup = field => {
+  if (!field) return null;
+  const compile = marksy({
+    createElement,
+    elements: {}
+  });
+  return compile(field).tree;
+};
 
   render() {
     if (!this.state.data) return null;
 
+    const content = getMarkup(this.state.data.content);
+
     return (
       <div>
         <Helmet title={this.state.data.title} />
         <h1>{this.state.data.title}</h1>
-        {this.state.data.content}
+        {content}
       </div>
     );
   }