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

examples/menu/CMakeLists.txt

@@ -0,0 +1,4 @@
+add_executable(menu_main
+  main.cpp
+)
+target_link_libraries(menu_main PRIVATE ftxui)

examples/menu/main.cpp

@@ -0,0 +1,21 @@
+#include <chrono>
+#include <iostream>
+#include <thread>
+
+#include "ftxui/screen_interactive.hpp"
+#include "ftxui/component/menu.hpp"
+
+int main(int argc, const char *argv[])
+{
+  ftxui::ScreenInteractive screen(30,3);
+  ftxui::component::Menu menu(screen.delegate());
+  menu.entries = {
+    L"entry 1",
+    L"entry 2",
+    L"entry 3"
+  };
+  menu.selected = 0;
+  menu.on_enter = screen.ExitLoopClosure();
+
+  screen.Loop();
+}