Add an UDL for combined hex RGB colors (#203)

In order to allow using the literal on its own it has been put into the
inline namespace `literals`.

examples/dom/style_color.cpp

@@ -28,7 +28,8 @@ int main(int argc, const char* argv[]) {
         color(Color::Red, text("Red")),
         color(Color::RedLight, text("RedLight")),
         color(Color::Yellow, text("Yellow")),
-        color(Color::YellowLight, text("YellowLight"))
+        color(Color::YellowLight, text("YellowLight")),
+        color(0x66ff66_rgb, text("Phosphor"))
       ),
       vbox(
         bgcolor(Color::Default, text("Default")),
@@ -47,7 +48,8 @@ int main(int argc, const char* argv[]) {
         bgcolor(Color::Red, text("Red")),
         bgcolor(Color::RedLight, text("RedLight")),
         bgcolor(Color::Yellow, text("Yellow")),
-        bgcolor(Color::YellowLight, text("YellowLight"))
+        bgcolor(Color::YellowLight, text("YellowLight")),
+        bgcolor(0x66ff66_rgb, text("Phosphor"))
       ),
       filler()
     );

include/ftxui/screen/color.hpp

@@ -322,6 +322,14 @@ class Color {
   uint8_t blue_ = 0;
 };
 
+inline namespace literals {
+
+/// @brief Creates a color from a combined hex RGB representation, 
+/// e.g. 0x808000_rgb
+Color operator""_rgb(unsigned long long int combined);
+
+}  // namespace literals
+
 }  // namespace ftxui
 
 #endif /* end of include guard: FTXUI_COLOR_H_ */

src/ftxui/screen/color.cpp

@@ -1,5 +1,7 @@
 #include "ftxui/screen/color.hpp"
 
+#include <cassert>
+
 #include "ftxui/screen/color_info.hpp"  // for GetColorInfo, ColorInfo
 #include "ftxui/screen/terminal.hpp"  // for Terminal, Terminal::Color, Terminal::Palette256, Terminal::TrueColor
 
@@ -143,6 +145,18 @@ Color Color::HSV(uint8_t h, uint8_t s, uint8_t v) {
   return Color(0, 0, 0);
 }
 
+inline namespace literals {
+
+Color operator""_rgb(unsigned long long int combined) {
+  assert(combined <= 0xffffffU);
+  auto const red = static_cast<uint8_t>(combined >> 16);
+  auto const green = static_cast<uint8_t>(combined >> 8);
+  auto const blue = static_cast<uint8_t>(combined);
+  return Color(red, green, blue);
+}
+
+}  // namespace literals
+
 }  // namespace ftxui
 
 // Copyright 2020 Arthur Sonzogni. All rights reserved.