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

examples/dom/size.cpp

@@ -10,7 +10,6 @@
 
 int main(int argc, const char* argv[]) {
   using namespace ftxui;
-  // clang-format off
   auto make_box = [](const std::wstring title) {
     return
       window(
@@ -26,7 +25,7 @@ int main(int argc, const char* argv[]) {
         | size(WIDTH, EQUAL, x)
     );
   }
-  // clang-format on
+
   auto document = hbox(std::move(content));
   auto screen = Screen::Create(Dimension::Fit(document));
   Render(screen, document.get());