Add support for nxxm.

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

src/ftxui/component/radiobox.cpp

@@ -0,0 +1,45 @@
+#include "ftxui/component/radiobox.hpp"
+#include <functional>
+
+namespace ftxui {
+
+Element RadioBox::Render() {
+  std::vector<Element> elements;
+  bool is_focused = Focused();
+  for (size_t i = 0; i < entries.size(); ++i) {
+    auto style =
+        (focused == int(i) && is_focused) ? focused_style : unfocused_style;
+    auto focus_management =
+        (focused != int(i)) ? nothing : is_focused ? focus : select;
+
+    const std::wstring& symbol = selected == int(i) ? checked : unchecked;
+    elements.push_back(hbox(text(symbol), text(entries[i]) | style) |
+                       focus_management);
+  }
+  return vbox(std::move(elements));
+}
+
+bool RadioBox::OnEvent(Event event) {
+  if (!Focused())
+    return false;
+
+  int new_focused = focused;
+  if (event == Event::ArrowUp || event == Event::Character('k'))
+    new_focused--;
+  if (event == Event::ArrowDown || event == Event::Character('j'))
+    new_focused++;
+  new_focused = std::max(0, std::min(int(entries.size()) - 1, new_focused));
+  if (focused != new_focused) {
+    focused = new_focused;
+    return true;
+  }
+
+  if (event == Event::Character(' ') || event==Event::Return) {
+    selected = focused;
+    on_change();
+  }
+
+  return false;
+}
+
+}  // namespace ftxui