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/gauge/main.cpp

@@ -2,15 +2,21 @@
 #include <iostream>
 #include <thread>
 
-#include "ftxui/core/screen.hpp"
-#include "ftxui/core/dom/elements.hpp"
+#include "ftxui/screen.hpp"
+#include "ftxui/dom/elements.hpp"
 
 int main(int argc, const char *argv[])
 {
   for(float percentage = 0; percentage <= 1.0; percentage+=0.001) {
+    std::wstring data_downloaded =
+        std::to_wstring(int(percentage * 44100)) + L"/44100";
     using namespace ftxui::dom;
     auto document =
-        hbox(text(L"gauge = -"), flex(gauge(percentage)), text(L"-"));
+        hbox(
+          text(L"downloading:"),
+          flex(gauge(percentage)),
+          text(L" " + data_downloaded)
+        );
     auto screen = ftxui::Screen(100, 1);
     Render(screen, document.get());
     std::cout << '\r' << screen.ToString() << std::flush;