Add focusable Renderer. (#173)

examples/component/CMakeLists.txt

examples/component/input.cpp

examples/component/renderer.cpp

include/ftxui/component/component.hpp

include/ftxui/screen/screen.hpp

src/ftxui/component/button.cpp

src/ftxui/component/checkbox.cpp

src/ftxui/component/input.cpp

src/ftxui/component/input_test.cpp

src/ftxui/component/menu.cpp

src/ftxui/component/radiobox.cpp

src/ftxui/component/renderer.cpp

src/ftxui/component/slider.cpp

src/ftxui/dom/clear_under.cpp

src/ftxui/dom/inverted.cpp

src/ftxui/dom/node_decorator.hpp

src/ftxui/dom/underlined.cpp

src/ftxui/dom/util.cpp

src/ftxui/screen/screen.cpp