Add support for nxxm.

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

include/ftxui/screen/color.hpp

@@ -0,0 +1,40 @@
+#ifndef FTXUI_SCREEN_COLOR
+#define FTXUI_SCREEN_COLOR
+
+#include <cstdint>
+
+namespace ftxui {
+
+enum class Color : uint8_t {
+  // --- Transparent -----
+  Default = 39,
+
+  // --- Grayscale -----
+  Black = 30,
+  GrayDark = 90,
+  GrayLight = 37,
+  White = 97,
+
+  // --- Hue -----
+  Blue = 34,
+  BlueLight = 94,
+
+  Cyan = 36,
+  CyanLight = 96,
+
+  Green = 32,
+  GreenLight = 92,
+
+  Magenta = 35,
+  MagentaLight = 95,
+
+  Red = 31,
+  RedLight = 91,
+
+  Yellow = 33,
+  YellowLight = 93,
+};
+
+}; // namespace ftxui
+
+#endif /* end of include guard: FTXUI_COLOR_H_ */