Add `expose_current_running_example_as` config option.

Fixes #963.
Ensure the current example can be exposed via multiple helper methods.
Isolate this spec from "leaking" changes to the helper module.
When using `expose_current_running…` no deprecation should be printed.
Improve example/running_example deprecation warning.
Add changelog entry.
Merge pull request #1135 from rspec/expose-current-running-example-2-99

Expose current running example 2 99