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/inverted.cpp

@@ -24,8 +24,8 @@ class Inverted : public NodeDecorator {
   }
 };
 
-std::unique_ptr<Node> inverted(Element child) {
-  return std::make_unique<Inverted>(unpack(std::move(child)));
+Element inverted(Element child) {
+  return std::make_shared<Inverted>(unpack(std::move(child)));
 }
 
 }  // namespace ftxui