Feature: strikethrough and underlinedDouble decorator. (#561)

This resolves:
https://github.com/ArthurSonzogni/FTXUI/issues/560

examples/dom/style_underlined_double.cpp

@@ -0,0 +1,25 @@
+#include <ftxui/dom/elements.hpp>  // for text, operator|, underlinedDouble, Fit, hbox, Element
+#include <ftxui/screen/screen.hpp>  // for Full, Screen
+#include <memory>                   // for allocator
+
+#include "ftxui/dom/node.hpp"      // for Render
+#include "ftxui/screen/color.hpp"  // for ftxui
+
+int main(int argc, const char* argv[]) {
+  using namespace ftxui;
+  auto document =  //
+      hbox({
+          text("This text is "),
+          text("underlinedDouble") | underlinedDouble,
+          text(". Do you like it?"),
+      });
+  auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
+  Render(screen, document);
+  screen.Print();
+
+  return 0;
+}
+
+// Copyright 2020 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.