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`.

include/ftxui/screen/color.hpp

@@ -2,7 +2,7 @@
 #define FTXUI_SCREEN_COLOR
 
 #include <stdint.h>  // for uint8_t
-#include <string>    // for wstring
+#include <string>  // for wstring
 
 #ifdef RGB
 // Workaround for wingdi.h (via Windows.h) defining macros that break things.
@@ -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_ */