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/text.cpp

@@ -36,8 +36,8 @@ class Text : public Node {
   std::wstring text_;
 };
 
-std::unique_ptr<Node> text(std::wstring text) {
-  return std::make_unique<Text>(text);
+Element text(std::wstring text) {
+  return std::make_shared<Text>(text);
 }
 
 }  // namespace ftxui