Feature: Dropdown options with callback (#826)

CHANGELOG.md

examples/component/CMakeLists.txt

examples/component/dropdown_custom.cpp

examples/component/homescreen.cpp

include/ftxui/component/component.hpp

include/ftxui/component/component_options.hpp

src/ftxui/component/button.cpp

src/ftxui/component/dropdown.cpp

src/ftxui/component/screen_interactive.cpp