Move heading anchors 50px up to avoid nav bar

Fixes #447.

We do this by moving the actual anchored element up in the page without moving the actual text. (Apple uses a similar trick in their framed docs.) Now this looks a bit sillier on smaller screens but it's better overall.

docs/_plugins/header_links.rb

@@ -11,7 +11,7 @@ class Redcarpet::Render::HTML
       .gsub(/\s+/, "-")
       .gsub(/[^A-Za-z0-9\-_.]/, "")
 
-    return "<h#{level} id=\"#{clean_title}\"><a href=\"##{clean_title}\">#{title}</a></h#{level}>"
+    return "<h#{level} id=\"#{clean_title}\" class=\"anchor\"><a href=\"##{clean_title}\">#{title}</a></h#{level}>"
   end
 end