Add support for single-file record packaging (`.wfdb`)

[tompollard]

As discussed in https://github.com/wfdb/wfdb-spec/issues/21, I think there is some support for creating an approach to allow WFDB records to be stored within a single file (e.g. 100.wfdb). I'd like to try adding support for this new file type, demoing first on a new branch.