Add colors.

.clang-format

examples/CMakeLists.txt

examples/color/CMakeLists.txt

examples/color/main.cpp

ftxui/CMakeLists.txt

ftxui/include/ftxui/color.hpp

ftxui/include/ftxui/dom/elements.hpp

ftxui/include/ftxui/screen.hpp

ftxui/src/ftxui/dom/bold.cpp

ftxui/src/ftxui/dom/color.cpp

ftxui/src/ftxui/dom/dim.cpp

ftxui/src/ftxui/dom/inverted.cpp

ftxui/src/ftxui/dom/node_decorator.cpp

ftxui/src/ftxui/dom/node_decorator.hpp

ftxui/src/ftxui/dom/underlined.cpp

ftxui/src/ftxui/screen.cpp

tutorial.md