Allow sidebar navs with custom sidebar content

Multiple sidebar navs can also be included on a page:

```
sidebar:
  - title: Navigation 1
    nav: nav1
  - title: Navigation 2
    nav: nav2
```

Fixes #1986

_includes/sidebar.html

@@ -14,6 +14,7 @@
       {% endif %}
       {% if s.title %}<h3>{{ s.title }}</h3>{% endif %}
       {% if s.text %}{{ s.text | markdownify }}{% endif %}
+      {% if s.nav %}{% include nav_list nav=s.nav %}{% endif %}
     {% endfor %}
     {% if page.sidebar.nav %}
       {% include nav_list nav=page.sidebar.nav %}

docs/_posts/2012-03-15-layout-sidebar-custom.md

@@ -9,6 +9,7 @@ sidebar:
     text: "Some text here."
   - title: "Another Title"
     text: "More text here."
+    nav: sidebar-sample
 ---
 
 This post has a custom sidebar set in the post's YAML Front Matter.

test/_posts/2012-03-15-layout-sidebar-custom.md

@@ -7,8 +7,7 @@ sidebar:
     image: http://placehold.it/350x250
     image_alt: "image"
     text: "Some text here."
-  - title: "Another Title"
-    text: "More text here."
+    nav: sidebar-sample
 ---
 
 This post has a custom sidebar set in the post's YAML Front Matter.