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/dbox.cpp

@@ -41,8 +41,8 @@ class DBox : public Node {
   }
 };
 
-std::unique_ptr<Node> dbox(Elements children) {
-  return std::make_unique<DBox>(std::move(children));
+Element dbox(Elements children) {
+  return std::make_shared<DBox>(std::move(children));
 }
 
 }  // namespace ftxui