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/toggle/main.cpp

@@ -0,0 +1,69 @@
+#include <chrono>
+#include <iostream>
+#include <thread>
+
+#include "ftxui/screen_interactive.hpp"
+#include "ftxui/component/toggle.hpp"
+#include "ftxui/component/component_horizontal.hpp"
+#include "ftxui/component/component_vertical.hpp"
+#include "ftxui/util/string.hpp"
+
+using namespace ftxui::component;
+using namespace ftxui::dom;
+
+class MyComponent : ComponentVertical {
+ public:
+  MyComponent(ftxui::component::Delegate* delegate)
+      : ComponentVertical(delegate),
+        toggle_1(delegate->NewChild()),
+        toggle_2(delegate->NewChild()),
+        toggle_3(delegate->NewChild()) {
+    toggle_1.on = L"On";
+    toggle_1.off = L"Off";
+
+    toggle_2.on = L"Enabled";
+    toggle_2.off = L"Disabled";
+
+    toggle_3.on = L"10€";
+    toggle_3.off = L"0€";
+
+    Focus(&toggle_1);
+  }
+
+  std::function<void()> on_enter = []() {};
+
+ private:
+  Toggle toggle_1;
+  Toggle toggle_2;
+  Toggle toggle_3;
+
+  Element Render() override {
+    return
+      vbox(
+        text(L"Choose your options:"),
+        text(L""),
+        hbox(text(L" * Poweroff on startup      : "), toggle_1.Render()),
+        hbox(text(L" * Out of process           : "), toggle_2.Render()),
+        hbox(text(L" * Price of the information : "), toggle_3.Render())
+      );
+  }
+
+  bool Event(int key) override {
+    if (ComponentVertical::Event(key))
+      return true;
+
+    if (key == 10) {
+      on_enter();
+      return true;
+    }
+
+    return false;
+  }
+};
+
+int main(int argc, const char* argv[]) {
+  ftxui::ScreenInteractive screen(50,5);
+  MyComponent component(screen.delegate());
+  component.on_enter = screen.ExitLoopClosure();
+  screen.Loop();
+}