Add <Subscriber quiet> prop to avoid invariant when rendered out of context

[mjackson]

Currently, if you render a <Subscriber> out of context it'll throw an invariant. This is similar to the context API itself, where if you try to use this.context.cupcakes outside the appropriate context you'll get an error.

What we don't have is a way for users to opt-out of the invariant when they're not rendered in the right context. This would be the equivalent of the isRequired functionality currently available to consumers of the context API through contextTypes.

The idea would be to add <Subscriber quiet> to opt-out of the invariant in these situations.

[mjackson]

I should also note that in the case a <Subscriber quiet> is rendered out of context its value should be undefined instead of null, same as this.context.cupcakes would be out of context.