Cache rendered blocks content to reuse them in further calls

Tests/StencilTests/fixtures/base-repeat.html

@@ -0,0 +1,5 @@
+{% block header %}Header{% endblock %}
+{% block body %}Body{% endblock %}
+Repeat
+{{ block.header }}
+{{ block.body }}

Tests/StencilTests/fixtures/child-repeat.html

@@ -0,0 +1,3 @@
+{% extends "base-repeat.html" %}
+{% block header %}Super_{{ block.super }} Child_Header{% endblock %}
+{% block body %}Child_Body{% endblock %}