use ndash as separator

display it only if both `page.title` and `site.title` are defined.

_layouts/base.html

@@ -27,8 +27,7 @@
 	<![endif]-->
 
   <title>
-  {% if page.title %}{{ page.title }}: {% endif %}
-  {% if site.title %}{{ site.title }}{% endif %}
+  {% if page.title %}{{ page.title }}{% endif %}{% if page.title and site.title %} &ndash; {% endif %}{% if site.title %}{{ site.title }}{% endif %}
   </title>  
 
   </head>