Add menu styles.

CMakeLists.txt

examples/CMakeLists.txt

examples/component/CMakeLists.txt

examples/component/menu2.cpp

examples/component/menu_style.cpp

examples/component/toggle.cpp

examples/dom/CMakeLists.txt

examples/dom/color.cpp

examples/dom/dbox.cpp

examples/dom/gauge.cpp

ftxui/CMakeLists.txt

ftxui/include/ftxui/component/menu.hpp

ftxui/include/ftxui/component/toggle.hpp

ftxui/include/ftxui/dom/elements.hpp

ftxui/src/ftxui/component/input.cpp

ftxui/src/ftxui/component/menu.cpp

ftxui/src/ftxui/component/toggle.cpp

ftxui/src/ftxui/dom/color.cpp

ftxui/src/ftxui/dom/dbox.cpp

ftxui/src/ftxui/dom/util.cpp