Use shared_ptr instead of unique_ptr for elements.

This allow users to pass it into initializer list. Then clang-format
will produce 'acceptable' indentations.

This fixes:
https://github.com/ArthurSonzogni/FTXUI/issues/18

src/ftxui/dom/node_decorator.hpp

@@ -5,7 +5,6 @@
 #ifndef FTXUI_DOM_NODE_DECORATOR_H_
 #define FTXUI_DOM_NODE_DECORATOR_H_
 
-#include "ftxui/dom/elements.hpp"
 #include "ftxui/dom/node.hpp"
 
 namespace ftxui {