Add breadcrumb navigation include

- Disable by default
- Add schema.org markup
- Add basic styling

_includes/breadcrumbs.html

@@ -0,0 +1,25 @@
+<nav class="breadcrumbs">
+  <ol itemscope itemtype="http://schema.org/BreadcrumbList">
+    {% assign crumbs = page.url | split: '/' %}
+    {% assign i = 1 %}
+    {% for crumb in crumbs offset: 1 %}
+      {% if forloop.first %}
+        <li itemprop="itemListElement" itemscope itemtype="http://schema.org/ListItem">
+          <a href="{{ absurl }}/" itemprop="item"><span itemprop="name">{{ site.breadcrumb_home_label }}</span></a>
+          <meta itemprop="position" content="{{ i }}" />
+        </li>
+        <span class="sep">{{ site.breadcrumb_separator }}</span>
+      {% endif %}
+      {% if forloop.last %}
+        <li class="current">{{ page.title }}</li>
+      {% else %}
+        {% assign i = i | plus: 1 %}
+        <li itemprop="itemListElement" itemscope itemtype="http://schema.org/ListItem">
+          <a href="{{ absurl }}{% assign crumb_limit = forloop.index | plus: 1 %}{% for crumb in crumbs limit: crumb_limit %}{{ crumb | append: '/' }}{% endfor %}" itemprop="item"><span itemprop="name">{{ crumb | replace: '-', ' ' | replace: '%20', ' ' | capitalize }}</span></a>
+          <meta itemprop="position" content="{{ i }}" />
+        </li>
+        <span class="sep">{{ site.breadcrumb_separator }}</span>
+      {% endif %}
+    {% endfor %}
+  </ol>
+</nav>