perf: postpone initializing site header

Postpone this step to the stage that DOM loaed, but img and css not

_includes/views/header.html

@@ -56,7 +56,7 @@
 </header>
 
 <script>
-  (function() {
+  function initHeader() {
     var lastScrollY = getScrollPos().y;
     var documentElement = document.documentElement;
 
@@ -87,5 +87,6 @@
     });
 
     storeScrollData();
-  })();
+  }
+  document.addEventListener('DOMContentLoaded', initHeader);
 </script>