Add support for nxxm.

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

src/ftxui/component/input.cpp

@@ -0,0 +1,86 @@
+#include "ftxui/component/input.hpp"
+#include "ftxui/screen/string.hpp"
+
+namespace ftxui {
+
+// Component implementation.
+Element Input::Render() {
+  cursor_position = std::max(0, std::min<int>(content.size(), cursor_position));
+  auto main_decorator = flex | size(HEIGHT, EQUAL, 1);
+  bool is_focused = Focused();
+
+  // Placeholder.
+  if (content.size() == 0) {
+    if (is_focused)
+      return text(placeholder) | dim | inverted | main_decorator;
+    else
+      return text(placeholder) | dim | main_decorator;
+  }
+
+  // Not focused.
+  if (!is_focused)
+    return text(content) | main_decorator;
+
+  std::wstring part_before_cursor = content.substr(0,cursor_position);
+  std::wstring part_at_cursor = cursor_position < (int)content.size()
+                                    ? content.substr(cursor_position, 1)
+                                    : L" ";
+  std::wstring part_after_cursor = cursor_position < (int)content.size() - 1
+                                       ? content.substr(cursor_position + 1)
+                                       : L"";
+  auto focused = 
+    is_focused ? focus : select;
+
+  return
+    hbox(
+      text(part_before_cursor),
+      text(part_at_cursor) | underlined | focused,
+      text(part_after_cursor)
+    ) | flex | inverted | frame | main_decorator;
+    
+}
+bool Input::OnEvent(Event event) {
+  cursor_position = std::max(0, std::min<int>(content.size(), cursor_position));
+  std::wstring c;
+
+  // Backspace.
+  if (event == Event::Backspace) {
+    if (cursor_position == 0)
+      return false;
+    content.erase(cursor_position - 1, 1);
+    cursor_position--;
+    return true;
+  }
+
+  // Enter.
+  if (event == Event::Return) {
+    on_enter();
+    return true;
+  }
+
+  if (event == Event::Custom) {
+    return false;
+  }
+
+  if (event == Event::ArrowLeft && cursor_position > 0) {
+    cursor_position--;
+    return true;
+  }
+
+  if (event == Event::ArrowRight && cursor_position < (int)content.size()) {
+    cursor_position++;
+    return true;
+  }
+
+  // Content
+  constexpr char ESC = char(27);
+  if (event.values[0] != ESC) {
+    wchar_t v = (char)event.values[0];
+    content.insert(cursor_position, 1, v);
+    cursor_position++;
+    return true;
+  }
+  return false;
+}
+
+}  // namespace ftxui