Add support for nxxm.

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

src/ftxui/dom/node.cpp

@@ -0,0 +1,47 @@
+#include "ftxui/dom/node.hpp"
+
+namespace ftxui {
+
+using ftxui::Screen;
+
+Node::Node() {}
+Node::Node(std::vector<std::unique_ptr<Node>> children)
+    : children(std::move(children)) {}
+Node::~Node() {}
+
+void Node::ComputeRequirement() {
+  for(auto& child : children)
+    child->ComputeRequirement();
+}
+
+void Node::SetBox(Box box) {
+  box_ = box;
+}
+
+void Node::Render(Screen& screen) {
+  for(auto& child : children)
+    child->Render(screen);
+}
+
+void Render(Screen& screen, Node* node) {
+  // Step 1: Find what dimension this elements wants to be.
+  node->ComputeRequirement();
+  
+  Box box;
+  box.x_min = 0;
+  box.y_min = 0;
+  box.x_max = screen.dimx() - 1;
+  box.y_max = screen.dimy() - 1;
+  
+  // Step 2: Assign a dimension to the element.
+  node->SetBox(box);
+  screen.stencil = box;
+
+  // Step 3: Draw the element.
+  node->Render(screen);
+
+  // Step 4: Apply shaders
+  screen.ApplyShader();
+}
+
+};  // namespace ftxui