My thoughts...

* As this currently exists, it doesn't accept IO objects, right?  It does `File.open` on the argument passed here, so the code assumes it is a file name.
* Supporting any IO object adds a nice bit of flexibility, although I'm having a hard time thinking of what people would use besides STDERR (the default) or a file.  Can you imagine folks using anything else?
* For the case of a file, it seems like a cleaner API to let people assign a file name rather than being forced to use `File.open` to make it an IO.
* For the "x deprecations logged to y" summary message printed at the end of the spec run, what would you use for `y` if it was just an IO?  I don't know that every IO object is properly "describable" (for lack of a better term) but if we restrict this to just filenames it's far simpler.

I think I lean towards filenames only for this.  Thoughts?