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/style_inverted.cpp

@@ -9,14 +9,11 @@
 
 int main(int argc, const char* argv[]) {
   using namespace ftxui;
-  // clang-format off
-  auto document =
-    hbox(
+  auto document = hbox({
       text(L"This text is "),
       text(L"inverted") | inverted,
-      text(L". Do you like it?")
-    );
-  // clang-format on
+      text(L". Do you like it?"),
+  });
   auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
   Render(screen, document.get());