Add support for nxxm.

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

src/ftxui/component/checkbox.cpp

@@ -0,0 +1,23 @@
+#include "ftxui/component/checkbox.hpp"
+#include <functional>
+
+namespace ftxui {
+
+Element CheckBox::Render() {
+  bool is_focused = Focused();
+  auto style = is_focused ? focused_style : unfocused_style;
+  auto focus_management = is_focused ? focus : state ? select : nothing;
+  return hbox(text(state ? checked : unchecked),
+              text(label) | style | focus_management);
+}
+
+bool CheckBox::OnEvent(Event event) {
+  if (event == Event::Character(' ') || event == Event::Return) {
+    state = !state;
+    on_change();
+    return true;
+  }
+  return false;  
+}
+
+}  // namespace ftxui