Feature: Add multi-line input. (#630)

CHANGELOG.md

cmake/ftxui_test.cmake

examples/component/CMakeLists.txt

examples/component/input_style.cpp

examples/component/textarea.cpp

include/ftxui/component/component.hpp

include/ftxui/component/component_options.hpp

include/ftxui/component/receiver.hpp

include/ftxui/screen/string.hpp

include/ftxui/util/ref.hpp

src/ftxui/component/button_test.cpp

src/ftxui/component/collapsible_test.cpp

src/ftxui/component/component_options.cpp

src/ftxui/component/component_test.cpp

src/ftxui/component/container_test.cpp

src/ftxui/component/hoverable_test.cpp

src/ftxui/component/input.cpp

src/ftxui/component/input_test.cpp

src/ftxui/component/radiobox_test.cpp

src/ftxui/component/receiver_test.cpp