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/inter.txt

@@ -0,0 +1,3 @@
+{% extends "base.txt" %}
+{% block title %}Inter {{ author }}{% endblock %}
+{% block body %}<main>Intermediate Content</main>{% endblock %}