fix: make open github in new tab secure

As target="_blank" offers some security problems there is a fix adding
rel="noopener noreferrer" too.

Signed-off-by: Conrad Zelck <git@simpel.cc>

index.html

@@ -16,7 +16,7 @@
     <nav>
       <h1>Sort Lines</h1>
       <a id="logo" class="cursordefault" href="/"><img src="../Simpel.png" alt="simpel icon" height="48" width="48"></a>
-      <a id="github" href="https://github.com/SimpelMe/sort-lines" target="_blank" title="watch source code">
+      <a id="github" href="https://github.com/SimpelMe/sort-lines" target="_blank" rel="noopener noreferrer" title="watch source code">
         <img id="github-cat" src="../github-cat-white.png" alt="github logo">
       </a>
     </nav>