add ability to insert additional global nav

_includes/additional_glboal_nav.html

@@ -0,0 +1 @@
+<!-- placeholder for additional global nav items -->

_includes/header.html

@@ -10,6 +10,7 @@
 					<li><a href="{{ site.baseurl | append: page.url }}">{{ page.title }}</a>
 				{% endif %}
 			{% endfor %}
+			{% include additional_glboal_nav.html %}
 		</ul>
 	</nav>
 </header>