Add Child Inheritance Feature (#198)

* add block and extend feature

* constify, code cleaning

* update single include

* add html language to readme

* clean tests

* constantify, update year

test/data/html-extend/template.txt

@@ -0,0 +1,7 @@
+{% extends "inter.txt" %}
+{% block head -%}
+    {{ super(2) }}
+    <style type="text/css">.important { color: #336699; }</style>
+{%- endblock %}
+{% block title %}{{ super(2) }}{{ super() }}: {{ title }}{% endblock %}
+{% block body %}<main>{{ content }}</main>{% endblock %}