Add support for nxxm.

[nxxm](https://nxxm.github.io)

include/ftxui/dom/node.hpp

@@ -0,0 +1,42 @@
+#ifndef FTXUI_DOM_NODE_HPP
+#define FTXUI_DOM_NODE_HPP
+
+#include <memory>
+#include <vector>
+
+#include "ftxui/dom/requirement.hpp"
+#include "ftxui/screen/box.hpp"
+#include "ftxui/screen/screen.hpp"
+
+namespace ftxui {
+
+class Node {
+ public:
+  Node();
+  Node(std::vector<std::unique_ptr<Node>> children);
+  virtual ~Node();
+
+  // Step 1: Compute layout requirement. Tell parent what dimensions this
+  //         element wants to be.
+  //         Propagated from Children to Parents.
+  virtual void ComputeRequirement();
+  Requirement requirement() { return requirement_; }
+
+  // Step 2: Assign this element its final dimensions.
+  //         Propagated from Parents to Children.
+  virtual void SetBox(Box box);
+
+  // Step 3: Draw this element.
+  virtual void Render(Screen& screen);
+
+  std::vector<std::unique_ptr<Node>> children;
+ protected:
+  Requirement requirement_;
+  Box box_;
+};
+
+void Render(Screen& screen, Node* node);
+
+};  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_DOM_NODE_HPP */