Change default value of `renderContainerOnSingleChild` to `true`

[barbogast]

I was very confused / surprised by the renderer not rendering my configured container all of a sudden. It took me a while to realise that the reason was that the block content only contained one element. I also nearly missed the option renderContainerOnSingleChild.

To have differing element hierarchies between content with one element and with multiple elements seems to be a special requirement rather then a good default. I suggest therefore to change the default of renderContainerOnSingleChild to true.

It would be a breaking change, though :-/

Thanks for considering :-)

[rexxars]

Closing this, as the new @portabletext/react library is the successor to this module. Please see https://github.com/portabletext/react-portabletext