Add support for nxxm.

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

src/ftxui/component/toggle.cpp

@@ -0,0 +1,42 @@
+#include "ftxui/component/toggle.hpp"
+
+namespace ftxui {
+
+Element Toggle::Render() {
+  bool is_focused = Focused();
+
+  Elements children;
+  for(size_t i = 0; i<entries.size(); ++i) {
+    // Separator.
+    if (i != 0)
+      children.push_back(separator());
+
+    // Entry.
+    auto style = (selected != int(i))
+                     ? normal_style
+                     : is_focused ? focused_style : selected_style;
+    auto focused = (selected != int(i)) ? nothing : is_focused ? focus : select;
+    children.push_back(text(entries[i]) | style | focused);
+  }
+  return hbox(std::move(children));
+}
+
+bool Toggle::OnEvent(Event event) {
+  if (selected > 0 &&
+      (event == Event::ArrowLeft || event == Event::Character('h'))) {
+    selected--;
+    on_change();
+    return true;
+  }
+
+  if (selected < int(entries.size()) - 1 &&
+      (event == Event::ArrowRight || event == Event::Character('l'))) {
+    selected++;
+    on_change();
+    return true;
+  }
+
+  return false;
+}
+
+}  // namespace ftxui