Add subtitle of article and page in <title> along with main title

Close #26

templates/article.html

@@ -1,7 +1,7 @@
 {% extends "base.html" %}
 
 {% block title %}
-{{ article.title }} · {{ super() }}
+{{ article.title }} {%if article.subtitle %} - {{ article.subtitle }} {% endif %} · {{ super() }}
 {% endblock title %}
 
 {% block head_description %}

templates/page.html

@@ -1,7 +1,7 @@
 {% extends "base.html" %}
 
 {% block title %}
-{{ page.title }} · {{ super() }}
+{{ page.title }} {%if page.subtitle %} - {{ page.subtitle }} {% endif %} · {{ super() }}
 {% endblock title %}
 
 {% block head_description %}