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

tutorial/build-with-cmake.md

@@ -18,13 +18,13 @@ main.cpp
 int main(int argc, const char *argv[]) {
   using namespace ftxui;
   auto document =
-    hbox(
+    hbox({
       text(L"left") | bold | border,
       text(L"middle") | flex | border,
-      text(L"right") | border
-    );
+      text(L"right") | border,
+      });
   auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
-  Render(screen, document.get());
+  Render(screen, document);
 
   std::cout << screen.ToString();