Build navigation list include helper

- Update navigation.yml to support multiple navigations and levels
- Optional include navigation into left sidebar

_includes/masthead.html

@@ -7,7 +7,7 @@
     <nav id="site-nav" class="greedy-nav">
       <button><div class="navicon"></div></button>
       <ul class="visible-links">
-        {% for link in site.data.navigation %}
+        {% for link in site.data.navigation.main %}
           {% if link.url contains 'http' %}
             {% assign domain = '' %}
             {% else %}