Support for filepath.Abs equivalent

[jmgilman]

We routinely use afero in our code to simplify writing unit tests against a filesystem. We recently had a use case where we needed to call filepath.Abs() to convert a relative path into an absolute one. The problem is that this function potentially calls out to os.Getwd() to assist in resolution, thus making it difficult to test.

Looking through the Afero API, it doesn't appear that there's an equivalent. Can support for this use case be added?