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/separator.cpp

@@ -9,20 +9,18 @@
 
 int main(int argc, const char* argv[]) {
   using namespace ftxui;
-  // clang-format off
-  auto document =
-    hbox(
-      text(L"left-column"),
-      separator(),
-      vbox(
-        center(text(L"right-top")) | flex,
-        separator(),
-        center(text(L"bottom-bottom"))
-      ) | flex,
-      separator(),
-      text(L"right-column")
-    ) | border;
-  // clang-format on
+  auto document = hbox({
+                      text(L"left-column"),
+                      separator(),
+                      vbox({
+                          center(text(L"right-top")) | flex,
+                          separator(),
+                          center(text(L"bottom-bottom")),
+                      }) | flex,
+                      separator(),
+                      text(L"right-column"),
+                  }) |
+                  border;
   auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
   Render(screen, document.get());