Made headings link to themselves.

Made each heading link to itself, by placing an <a> tag inside each
<h[1-6]> tag.

Also adds class="header" to each header, allowing for the CSS rule
    .header a { ...  }
to remove all styling from the inner links, allowing the outputed
document to look like before.

autoload/vimwiki/style.css

@@ -25,6 +25,7 @@ del {text-decoration: line-through; color: #777777;}
 .justcenter {text-align: center;}
 .center {margin-left: auto; margin-right: auto;}
 .tag {background-color: #eeeeee; font-family: monospace; padding: 2px;}
+.header a {text-decoration: none; color: inherit;}
 
 /* classes for items of todo lists */
 .rejected {