Add support for finding load time warnings

Extraction of the supporting code from rspec/rspec-core#1654.
If we're that worried, we should move this code out of lib and put it in a sister dir of `lib` and then add it to the load path ourselves.  I consider that out of scope for this PR, though.  And overall, I'm not really worried...if users want to require private undocumented code when we've said that the APIs in this library are private, they can't complain when we change things.