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 %}

_includes/nav_list.html

@@ -0,0 +1,24 @@
+{% include base_path %}
+{% assign navigation = site.data.navigation[page.sidebar.nav] %}
+
+<nav>
+  <ul>
+    {% for nav in navigation %}
+      <li>{{ nav.title }}
+        {% if nav.children != null %}
+        <ul>
+          {% for child in nav.children %}
+            {% assign nav_url = child.path | prepend: "/" | prepend: page.sidebar.nav | prepend: "/" | append: "/" | prepend: base_path %}
+            {% if nav_url contains page.url %}
+              {% assign c = "current" %}
+            {% else %}
+              {% assign c = "" %}
+            {% endif %}
+            <li><a href="{{ nav_url }}" class="{{ c }}">{{ child.title }}</a></li>
+          {% endfor %}
+        </ul>
+        {% endif %}
+      </li>
+    {% endfor %}
+  </ul>
+</nav>

_includes/sidebar.html

@@ -15,6 +15,9 @@
       {% if s.title %}<h3>{{ s.title }}</h3>{% endif %}
       {% if s.text %}{{ s.text | markdownify }}{% endif %}
     {% endfor %}
+    {% if page.sidebar.nav %}
+      {% include nav_list.html items=page.sidebar.nav %}
+    {% endif %}
   {% endif %}
   </div>
 {% endif %}