The `output` arg detection is clever :).

Anyhow, should this also issue a deprecation in the following situation?

* The user has configured `color = true` (so `color` will return `true` in RSpec 3)
* `output_stream` has been configured to a non-tty.

Under these conditions, I believe the value of `color` will be different on RSpec 3 (where it'll return `true` because it only considers user config) than here (where it'll be `false` because of the non-tty output).  Given the behavioral change, we should probably issue a deprecation warning.

Actually...while we technically probably should, I just realized that there's no public API available in 2.99 that provides the behavior of `color` on 3.0 so there's nothing to suggest the user to use instead to keep the same behavior that they are relying on.  So I'm thinking we don't bother for this case since it's such an edge case anyway.