Allow to specifiy a container and a default component to load when stream is not ready

[nmondon]

[nmondon]

The container is really not needed as we currently don't have a container...

OK for the default component, which can be replaced by a startWith value in the combinator observable but it's not convenient

[nmondon]

ok, not tested yet,
also, we should add a key="waiting" in order to enable nice transitions as react-addons-css-transition-group is waiting for a key