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`.

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.