add rel noopener for anchor tags with _blank (close hasura#39)

Author: spezzino

src/components/mdxComponents/anchor.js

@@ -3,7 +3,7 @@ import * as React from "react";
 const AnchorTag = ({ children: link, ...props }) => {
   if(link) {
     return (
-      <a href={props.href} target="_blank">{link}</a>
+      <a href={props.href} target="_blank" rel="noopener">{link}</a>
     );
   } else {
     return null;