Add Subresource Integrity (SRI) support (#164)

_includes/head.html

@@ -17,7 +17,7 @@
 
   {% if layout.common-ext-css %}
     {% for css in layout.common-ext-css %}
-      <link rel="stylesheet" href="{{ css }}" />
+      {% include ext-css.html css=css %}
     {% endfor %}
   {% endif %}
 
@@ -35,7 +35,7 @@
 
   {% if page.ext-css %}
     {% for css in page.ext-css %}
-      <link rel="stylesheet" href="{{ css }}" />
+      {% include ext-css.html css=css %}
     {% endfor %}
   {% endif %}