Add a config option to print panic output to stdout.

[alercah]

I'm working on a "testing harness lite" library, where multiple erros can be accumulated and then emitted all at the end of the test.

I would like to make it so that "primary" test output goes to stdout, while logging output goes to stderr. This creates an unusual case where panics are "primary" and therefore should got to stdout rather than the usual stderr.

So I would like a config option that supports changing the target of the panic hook's printing to stdout instead of stderr.