Implement a lot of new features.

This commit deserve to be cut into at least 8 sub commit. Sorry, I
acknowledge this is bad... Here are the new features:

 * dom decorator: bold, dim, underlined, inverted.
 * component mechanism
 * components
   * menu
   * toogle

ftxui/include/ftxui/dom/node.hpp

@@ -0,0 +1,44 @@
+#ifndef DOM_NODE_HPP
+#define DOM_NODE_HPP
+
+#include <memory>
+#include <vector>
+
+#include "ftxui/requirement.hpp"
+#include "ftxui/screen.hpp"
+#include "ftxui/box.hpp"
+
+namespace ftxui {
+namespace dom {
+
+class Node {
+ public:
+  Node();
+  Node(std::vector<std::unique_ptr<Node>> children);
+  virtual ~Node();
+
+  // Step 1: Direction parent <= children.
+  //         Compute layout requirement. Tell parent what dimensions this
+  //         element wants to be.
+  virtual void ComputeRequirement();
+  Requirement requirement() { return requirement_; }
+
+  // Step 2: Direction parent => children.
+  //         Assign this element its final dimensions.
+  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 dom
+};  // namespace ftxui
+
+#endif /* end of include guard: DOM_NODE_HPP */