Feature: `strikethrough` and `underlinedDouble` decorator. (#561)

CHANGELOG.md

CMakeLists.txt

README.md

doc/mainpage.md

examples/dom/CMakeLists.txt

examples/dom/style_gallery.cpp

examples/dom/style_strikethrough.cpp

examples/dom/style_underlined_double.cpp

include/ftxui/dom/elements.hpp

include/ftxui/screen/screen.hpp

src/ftxui/dom/strikethrough.cpp

src/ftxui/dom/underlined_double.cpp

src/ftxui/screen/screen.cpp