Add support for nxxm.

[nxxm](https://nxxm.github.io)

src/ftxui/component/menu.cpp

@@ -0,0 +1,46 @@
+#include "ftxui/component/menu.hpp"
+#include <algorithm>
+#include <iostream>
+
+namespace ftxui {
+
+Element Menu::Render() {
+  std::vector<Element> elements;
+  bool is_focused = Focused();
+  for (size_t i = 0; i < entries.size(); ++i) {
+    auto style = (selected != int(i))
+                     ? normal_style
+                     : is_focused ? focused_style : selected_style;
+    auto focused = (selected != int(i)) ? nothing : is_focused ? focus : select;
+    auto icon = (selected != int(i)) ? L"  " : L"> ";
+    elements.push_back(text(icon + entries[i]) | style | focused);
+  }
+  return vbox(std::move(elements));
+}
+
+bool Menu::OnEvent(Event event) {
+  if (!Focused())
+    return false;
+
+  int new_selected = selected;
+  if (event == Event::ArrowUp || event == Event::Character('k'))
+    new_selected--;
+  if (event == Event::ArrowDown || event == Event::Character('j'))
+    new_selected++;
+  new_selected = std::max(0, std::min(int(entries.size()) - 1, new_selected));
+
+  if (selected != new_selected) {
+    selected = new_selected;
+    on_change();
+    return true;
+  }
+
+  if (event == Event::Return) {
+    on_enter();
+    return true;
+  }
+
+  return false;
+}
+
+}  // namespace ftxui