Add support for nxxm.

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

src/ftxui/dom/color.cpp

@@ -0,0 +1,61 @@
+#include "ftxui/dom/node_decorator.hpp"
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+class BgColor : public NodeDecorator {
+ public:
+  BgColor(Elements children, Color color)
+      : NodeDecorator(std::move(children)), color_(color) {}
+
+  void Render(Screen& screen) override {
+    for (int y = box_.y_min; y <= box_.y_max; ++y) {
+      for (int x = box_.x_min; x <= box_.x_max; ++x) {
+        screen.PixelAt(x, y).background_color = color_;
+      }
+    }
+    NodeDecorator::Render(screen);
+  }
+
+  Color color_;
+};
+
+class FgColor : public NodeDecorator {
+ public:
+  FgColor(Elements children, Color color)
+      : NodeDecorator(std::move(children)), color_(color) {}
+  ~FgColor() override {}
+
+  void Render(Screen& screen) override {
+    for (int y = box_.y_min; y <= box_.y_max; ++y) {
+      for (int x = box_.x_min; x <= box_.x_max; ++x) {
+        screen.PixelAt(x, y).foreground_color = color_;
+      }
+    }
+    NodeDecorator::Render(screen);
+  }
+
+  Color color_;
+};
+
+std::unique_ptr<Node> color(Color c, Element child) {
+  return std::make_unique<FgColor>(unpack(std::move(child)), c);
+}
+
+std::unique_ptr<Node> bgcolor(Color c, Element child) {
+  return std::make_unique<BgColor>(unpack(std::move(child)), c);
+}
+
+Decorator color(Color c) {
+  return [c](Element child) {
+    return color(c, std::move(child));
+  };
+}
+
+Decorator bgcolor(Color c) {
+  return [c](Element child) {
+    return bgcolor(c, std::move(child));
+  };
+}
+
+};  // namespace ftxui