Feature: Option to show observable results in a log

GraphiQL supports both promise and observable fetchers, which let us really easily add support for GraphQL subscriptions. However, this approach just shows the most recent subscription result.

If there was an option somewhere to show a log of all observable results instead of just the most recent one, that would be super useful! I'm not sure if it should be:

1. The default (given subscriptions might be more common than live queries)
2. A UI option that appears if your fetcher returns an observable
3. A prop passed to the component

Or something else.