Color alpha support. (#884)

CHANGELOG.md

include/ftxui/screen/color.hpp

include/ftxui/screen/pixel.hpp

src/ftxui/dom/clear_under.cpp

src/ftxui/dom/color.cpp

src/ftxui/dom/dbox.cpp

src/ftxui/screen/color.cpp

src/ftxui/screen/screen.cpp