Add hide_title metadata that hides the title text on the top of the doc (#540)

lib/core/Doc.js

@@ -60,11 +60,12 @@ class Doc extends React.Component {
         </a>
       );
     }
+
     return (
       <div className="post">
         <header className="postHeader">
           {editLink}
-          <h1>{this.props.title}</h1>
+          {!this.props.hideTitle && <h1>{this.props.title}</h1>}
         </header>
         <article>
           <MarkdownBlock>{this.props.content}</MarkdownBlock>