Add #to_io and #reopen methods to `StdErrSplitter`

These changes are required to support capturing the `$stderr` stream in a tempfile rather than a `StringIO`. See https://github.com/rspec/rspec-expectations/pull/700 for the rationale.
Does the `reset!` call have to wait until after stderr has been reopened?  If not, this can be simplified to:

``` ruby
reset!
@orig_stderr.reopen(*args)
```