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

include/inja/node.hpp

@@ -1,4 +1,4 @@
-// Copyright (c) 2020 Pantor. All rights reserved.
+// Copyright (c) 2021 Pantor. All rights reserved.
 
 #ifndef INCLUDE_INJA_NODE_HPP_
 #define INCLUDE_INJA_NODE_HPP_
@@ -28,6 +28,8 @@ class ForArrayStatementNode;
 class ForObjectStatementNode;
 class IfStatementNode;
 class IncludeStatementNode;
+class ExtendsStatementNode;
+class BlockStatementNode;
 class SetStatementNode;
 
 
@@ -48,6 +50,8 @@ public:
   virtual void visit(const ForObjectStatementNode& node) = 0;
   virtual void visit(const IfStatementNode& node) = 0;
   virtual void visit(const IncludeStatementNode& node) = 0;
+  virtual void visit(const ExtendsStatementNode& node) = 0;
+  virtual void visit(const BlockStatementNode& node) = 0;
   virtual void visit(const SetStatementNode& node) = 0;
 };
 
@@ -331,6 +335,30 @@ public:
   }
 };
 
+class ExtendsStatementNode : public StatementNode {
+public:
+  const std::string file;
+
+  explicit ExtendsStatementNode(const std::string& file, size_t pos) : StatementNode(pos), file(file) { }
+
+  void accept(NodeVisitor& v) const {
+    v.visit(*this);
+  };
+};
+
+class BlockStatementNode : public StatementNode {
+public:
+  const std::string name;
+  BlockNode block;
+  BlockNode *const parent;
+
+  explicit BlockStatementNode(BlockNode *const parent, const std::string& name, size_t pos) : StatementNode(pos), parent(parent), name(name) { }
+
+  void accept(NodeVisitor& v) const {
+    v.visit(*this);
+  };
+};
+
 class SetStatementNode : public StatementNode {
 public:
   const std::string key;