Add expose_current_running_example_as config option.

[myronmarston]

Fixes #963.

[myronmarston]

Anyone want to review this? /cc @xaviershay @JonRowe @samphippen @alindeman @soulcutter

[soulcutter]

Looks good to me. Might be worth documenting in cucumber.

[fables-tales]

I agree with @soulcutter a cucumber would be useful as this ends up being a user facing option.

[myronmarston]

Thanks, @soulcutter and @samphippen!

I just pushed two more small commits -- just some improvements to the spec.

As for docing it with a cuke: I'd actually rather not give it that kind of visibility. This config option isn't one I'd normally recommend you use. It exists purely to assist with upgrades from 2.x where the user depends on some gem that uses the old example API. I'm planning to backport this to 2.99 and mention this in the deprecation notice issued from RSpec::Core::ExampleGroup#example and RSpec::Core::ExampleGroup#running_example, which will give it visibility to exactly the set of users this is intended for.

[soulcutter]

I did wonder how visible this configuration option was meant to be :) That explains it!

Good catch about leaking the Module changes.