Use assets plugin to minify CSS

Fix #67

templates/_includes/minify_css.html

@@ -0,0 +1,4 @@
+{% assets filters="cssmin", output="css/style.min.css", "css/elegant.css", "css/custom.css" %}
+<link rel="stylesheet" href="{{ SITEURL }}/{{ ASSET_URL }}">
+{% endassets %}
+

templates/base.html

@@ -25,9 +25,13 @@
         {% block head_links %}
         <link href="//netdna.bootstrapcdn.com/twitter-bootstrap/2.3.2/css/bootstrap-combined.min.css" rel="stylesheet">
         <link href="//netdna.bootstrapcdn.com/font-awesome/4.0.1/css/font-awesome.css" rel="stylesheet">
+        {% if 'assets' in PLUGINS %}
+        {% include '_includes/minify_css.html' with context %}
+        {% else %}
         <link rel="stylesheet" type="text/css" href="{{ SITEURL }}/theme/css/elegant.css" media="screen">
-        {%if CUSTOM_CSS %}
+            {%if CUSTOM_CSS %}
         <link rel="stylesheet" type="text/css" href="{{ SITEURL }}/theme/css/custom.css" media="screen">
+            {% endif %}
         {% endif %}
         {% endblock head_links %}
         {% include '_includes/favicon_links.html' %}